The primary objective is to evaluate the analgesic efficacy and safety of IV YZJ-4729 Tartrate Injection in patients with acute postoperative pain following abdominal surgery.

Inclusion Criteria:

1. Be able to understand the study purpose and cooperate with the study procedures of this trial, voluntarily provide written informed consent
2. Body mass index (BMI) ≥18 kg/m2 and ≤28 kg/m2.
3. ASA I - II
4. Experiences a pain intensity rating of moderate to severe acute pain following abdominal surgery, with a pain intensity score of ≥4 on a Numeric Rating Scale (NRS) within 4 hours post-surgery

Exclusion Criteria:

1. Allergy to opioids and other medications that may be used during the trial.
2. Nervous system disorders (e.g. epilepsy)
3. Psychiatric disorders (e.g. depression)
4. History of difficult airways
5. Random blood glucose ≥11.1 mmol/L
6. Uncontrolled blood pressure [e.g., hypertension (systolic blood pressure ≥160 mmHg, diastolic blood pressure ≥100 mmHg)，or hypotension(systolic blood pressure <90 mmHg)]
7. Abnormal pulse oxygen saturation (SpO2 <90 %)
8. Abnormal liver function or renal function
9. Used agents that could affect the analgesic response
10. Used agents that could affect drug metabolism
11. Has previously participated in another YZJ-4729 clinical study