To evaluate the effect of dexmedetomidine as an adjuvant of ropivacaine in the TAP block on cesarean section parturients under multimodal analgesia, optimize the multimodal analgesia program for cesarean section, and guide perioperative analgesia managemen。This is a single center, double-blind, randomized clinical trial.

Inclusion Criteria:

• 1) 37-42 weeks of gestation

  2) Plan cesarean section

  3) Receiving patient controlled intravenous analgesia

  4) Age>18 years

  5) ASA（American Society of Anesthesiologists) grade I-III

  6) Voluntary participation and informed consent

Exclusion Criteria:

• 1)The anesthesia method for cesarean section is general anesthesia or intraspinal anesthesia, and epidural administration is used

  2) Combined with other opioids during operation

  3) High risk pregnancy (multiple pregnancy, in vitro pregnancy, etc.) or pregnancy related complications (hypertension, preeclampsia, chorioamnionitis, etc.)

  4) Times of previous cesarean section ≥ 3

  5) BMI ≥ 50kg/m2 is not suitable for TAP block

  6) Allergies or contraindications to the drugs involved in the study

  7) Combined with operations other than tubal ligation and ovariectomy

  8) Severe renal function impairment (SCR>176 µ mol/L and/or blood urea nitrogen>17.9 mmol/L), severe liver function impairment (ALT and/or aspartate aminotransferase exceed 3 times the upper limit of normal value)

  9) Increased risk of coagulation dysfunction or bleeding (PLT<80 × 109/L or international normalized ratio> 1.5)

  10) History of chronic pain or opiate abuse

  11) Other clinical trials in the last three months