This is a multi-center, randomized, double-blind, placebo/active-controlled study. 200 subjects undergoing orthopedic 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.