This is a multi-center, randomized, double-blind, double-dummy, placebo/active-controlled study. 200 subjects undergoing abdominal surgery under general anesthesia are planned to be enrolled and randomized into the HSK55718 group, morphine group, and placebo group.

Inclusion Criteria:

• Age ≥18 years old, regardless of gender;
• 18 kg/m2≤BMI≤30 kg/m2;
• American Society of Anesthesiologists (ASA) gradeⅰ-ⅲ;
• Accept abdominal surgery under general anesthesia;
• Within 4 hours after the end of surgery, NRS≥4 in the resting state at any time;
• Fully understand and voluntarily participate this trial, and sign the informed consent form;

Exclusion Criteria:

• A history of cardiovascular, respiratory, neurological, or psychiatric diseases that meet the protocol-defined exclusion criteria.
• Clinically significant abnormalities in laboratory tests during the screening period.
• A history of severe drug allergy or known allergy to the investigational drug.
• Continuous use of opioid analgesics for more than 10 days for any reason within 3 months prior to randomization.
• Use of any medication that may affect analgesia, where the last dose was administered less than 5 half-lives of that drug before randomization.
• A history of drug abuse, substance abuse, and/or alcohol abuse within the past year.
• Pregnant or lactating women.
• Participation in another drug clinical trial within 3 months prior to the screening period.
• Presence of any sensory dysfunction.
• Presence of any painful physical condition.
• Subjects with any other factor considered by the investigator to be ineligible for participation in the trial.