Objective:To investigate the effect of ultrasound-guided serratus anterior plane block on perioperative analgesia in patients after modified radical mastectomy(MRM) and to explore its role in preventing post-mastectomy pain syndrome(PMPS). Methods:One hundred female patients scheduled for modified radical mastectomy,aged 28 to 75,were randoly divided into 2 groups the general anesthesia group(G) and the general anesthesia group combined with nerve block group(GB). Before induction of anesthesia,patients in the GB group were injected with 0.33% ropivacaine for 30 mL in the serratus anterior plane under ultrasound guidance. Patients in the group G were injected with the same amount of saline at the same location. Patients in the two groups underwent tracheal intuba-tion,followed by static suction combined anesthesia. Patients in both groups were given patient controlled intravenous analgesia(PCIA) after surgery. The times of effective delivered doses 48 h postoperatively,the cases of remedial analgesia were recorded. Nu-merical rating scale(NRS) was used to evaluated the pain severity at 4 hours,12 hours,24 hours,and 48 hours postoperatively and at 3 months,6 months,9 months,12 months postoperatively in both groups. PMPS incidence will also be recorded. Results:①The NRS scores at rest in GB group were significantly lower than that of the G group at 4 h(P=0.000),12 h(P=0.000),24 h(P=0.000),and 48 h(P=0.001) postoperatively;NRS scores at movement in in GB group were significantly lower than that of the G group at 4 h(P=0.000),12 h(P=0.000),24 h(P=0.002),and 48 h(P=0.001) postoperatively. ②The number of effective analgesia pump pressing within 48 h in GB(0.07±0.34) was signifi-cantly less than that in the G group(0.56±0.82)(P=0.001);the remedial analgesic ratio in GB group(4.77%) was lower than G group(35.90%)(P=0.000). ③The NRS scores in the GB group at 6(P=0.020) and 9 months(P=0.016) after surgery were significantly lower than those in the G group. ④The incidence of PMPS in GB group at 6 months(47.62%) and 9 months(61.54%) postopera-tively was significantly lower than that in the G group at 6 months(71.80%,P=0.041) and 9 months(61.54%,P=0.020)postopera-tively. ⑤There was a correlation between the acute NRS scores and the chronic NRS scores. Conclusion:Ultrasound-guided serratus anterior plane block applied to modified radical mastectomy for breast cancer can not only improve acute pain scores,reduce periop-erative opioid consumption,but also reduce the incidence and severity of PMPS. In addition,the postoperative acute pain NRS score was related to the severity of PMPS.