High-definition transcranial electrical stimulation (HD-tES) is a non-invasive brain neuromodulation technique that applies a small electrical current to the scalp to alter neural excitability and stimulate localized brain activation. Previous clinical trials have explored the use of HD-tES for treating mental health conditions such as depression, anxiety, obsessive-compulsive disorder, and post-traumatic stress disorder. This trial aims to investigate the efficacy and safety of HD-tES in ameliorating anxiety symptoms among patients with generalized anxiety disorder (GAD), thereby validating its potential as a treatment for anxiety disorders.

Participants will be randomly assigned to one of four HD-tES treatment groups: (1) HD-tES inhibitory waveform (cDC+cTBS) applied to the right dorsolateral prefrontal cortex (DLPFC) for 10 minutes, followed by sham excitatory waveform (aDC+iTBS) stimulation applied to the left DLPFC for 10 minutes. (2) Sham inhibitory waveform (cDC+cTBS) stimulation applied to the right DLPFC for 10 minutes, followed by HD-tES excitatory waveform (aDC+iTBS) applied to the left DLPFC for 10 minutes. (3) HD-tES inhibitory waveform (cDC+cTBS) applied to the right DLPFC for 10 minutes, followed by HD-tES excitatory waveform (aDC+iTBS) applied to the left DLPFC for 10 minutes. (4) Sham inhibitory waveform (cDC+cTBS) stimulation applied to the right DLPFC for 10 minutes, followed by sham excitatory waveform (aDC+iTBS) stimulation applied to the left DLPFC for 10 minutes. Regardless of the group assignment, participants will undergo treatment sessions over a 2-week period, with five sessions per week and no more than one session per day. Each session lasts approximately 20 minutes. Assessments will be conducted before the treatment, weekly during the treatment period (at the end of the first and second weeks), and a follow-up evaluation will be performed one week after the conclusion of the treatment.

Inclusion Criteria:

• Age between 18 and 65 years.
• Diagnosed with Generalized Anxiety Disorder (GAD) by a psychiatrist according to DSM-5 criteria.
• Hamilton Anxiety Rating Scale (HAM-A) score ≥ 14.
• Hamilton Depression Rating Scale (HAM-D; 17-item version) score ≤ 17.
• Has been consistently receiving psychological counseling and/or medication with stable types and dosages for at least 6 weeks prior to enrollment; or is deemed unsuitable for medication and/or psychological counseling; or refuses medication and/or psychological counseling.

Exclusion Criteria:

• Received rTMS or any other form of non-invasive brain stimulation techniques within 2 weeks prior to the study and during the study period.
• Presence of severe neurological disorders (e.g., stroke, brain tumor, epilepsy, organic brain diseases) or psychiatric disorders (e.g., schizophrenia and other psychotic disorders, bipolar disorder, obsessive-compulsive disorder, other types of anxiety disorders, substance abuse).
• Severe or unstable physiological conditions that may affect the autonomic or central nervous system (e.g., acute gastrointestinal diseases, cardiovascular diseases, thyroid disorders).
• History of cardiac arrhythmia.
• Presence of implanted medical electronic devices (e.g., pacemakers).
• Presence of metallic implants in the head or neck region.
• Open wounds on the scalp at the site of electrode contact.
• History of head surgery or significant head trauma that, based on physician evaluation, makes the individual unsuitable for inclusion.
• Individuals with significant suicide risk (HAM-D Item 3 score on suicidal risk ≥ 3).
• Presence of immune disorders (e.g., systemic lupus erythematosus, rheumatoid arthritis, ankylosing spondylitis, inflammatory bowel diseases).
• Individuals with abnormal or heightened sensitivity to electrical stimulation, making them unable to tolerate it.
• Pregnancy (for female participants: must be postmenopausal or surgically sterilized. Females of childbearing potential must have a negative pregnancy test. Female participants capable of becoming pregnant and their male partners with female partners capable of becoming pregnant must agree to use effective contraception during the trial and for 4 months after the last study intervention, such as oral contraceptives, dual barrier methods, or intrauterine devices, or agree to abstain from sexual activity during this period. Non-childbearing females are defined as those who have undergone bilateral oophorectomy or are postmenopausal).
• Taking medications that lower the seizure threshold.
• Alcohol or substance abuse.
• Convexity skull defects or elevated intracranial pressure.
• Breastfeeding women.
• Other conditions deemed unsuitable for transcranial electrical stimulation based on physician evaluation.