The goal of this study is to use a pilot RCT to assess the effectiveness of Written Exposure Therapy (WET) and a mindfulness-based app (MBA) for managing PTSD and comorbid insomnia in China. The main questions it aims to answer are:

1. Does WET alone reduce PTSD symptoms in Chinese patients with PTSD and subthreshold PTSD?
2. Does the addition of MBA to WET lead to greater reductions in comorbid insomnia symptoms compared to WET alone?

Researchers will compare WET alone and MCC to see if WET provides benefits in managing PTSD. Researchers will compare WET plus MBA and WET group to see if the integrated MBA treatment provides additional benefits in managing insomnia.

Participants will:

• Undergo random assignment to one of three groups: WET, WET plus MBA, or MCC.
• Receive clinical interviews for primary outcomes (PTSD symptoms) at baseline, posttreatment, and during follow-ups.
• Provide self-reported data on PTSD symptoms and insomnia severity at multiple time points.
• In the WET plus MBA group, additionally use a mindfulness-based app as part of their treatment.

Inclusion Criteria:

1. Adult male and female aged between 18-65 years old;
2. A diagnosis of PTSD or subthreshold PTSD (PTSD Symptom Scale, Interview Version for DSM-5, PSSI-5);
3. If currently taking psychotropic medication, taking a stable one for at least 4 weeks;
4. A smartphone owner and no obstacle to the Now Meditation App;
5. Scoring 12 or more on Insomnia Severity Index (ISI; the symptoms appear after the diagnosis of full/subthreshold PTSD).

Exclusion Criteria:

1. A diagnosis of bipolar disorder or psychotic disorder;
2. Current substance dependence;
3. Evidence of a moderate or severe traumatic brain injury (as determined by the obstacle to comprehending the baseline screening questionnaires);
4. Serious suicidal ideation (as determined by the Scale for Suicidal Ideation);
5. Other psychiatric disorders severe enough to warrant designation as the primary disorder;
6. Taking psychotherapy for PTSD currently.