This is a multi-center, randomized, double-blind, placebo/active-controlled study. 160 subjects undergoing orthopedic surgery under general anesthesia are planned to be enrolled and randomized into the HSK21542 group, morphine group, and placebo group.

Inclusion Criteria:

• 18 years old ≤ age ≤74 years old, regardless of gender;
• 18 kg/m2≤BMI≤32 kg/m2, meet the weight range specified in the protocol;
• American Society of Anesthesiologists (ASA) grade 1-3;
• Accept hip replacement 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:

• Patients with allergy to opioids or any component of the trial drug;
• Patients with a history of severe cardiovascular and cerebrovascular diseases, ,and psychiatric disorders;
• Last use of opioid or non-opioid analgesics less than 5 half-lives or the duration of drug efficacy prior to randomization; use of drugs with unclear half-lives that may affect analgesic efficacy within 7 days prior to randomization, or last use of drugs affecting analgesic efficacy less than 5 half-lives prior to randomization, as well as Chinese herbal medicines or proprietary Chinese medicines deemed by the investigator as potentially impacting the evaluation of efficacy and safety;
• Continuous use of opioid analgesics for more than 7 days for any reason within 30 days prior to screening;
• Laboratory test results during the screening period meeting any of the following criteria: 1) Significantly abnormal hematology results; 2) Significantly abnormal prothrombin time test results; 3) Significantly abnormal liver or kidney function; 4) Significantly abnormal fasting blood glucose;
• Positive test results during the screening period for hepatitis B surface antigen (HBsAg), hepatitis C antibody (HCVAb), syphilis antibody, or human immunodeficiency virus (HIV) antibody;
• A history of drug, drug, and/or alcohol abuse
• Participate in any clinical trail within 30 days;
• Pregnant or lactating female subjects;
• Subjects of childbearing potential who are unwilling to use contraception throughout the study period or plan to become pregnant within 3 months after the study.
• Subjects with any other factor considered by the investigator to be ineligible for participation in the trial.