Objective To investigate the effect of OPRM1 118G gene polymorphism on postoperative analgesia after intrasheath morphine injection in cesarean section. Methods From January 2017 to January 2018, 194 cases of women underwent elective cesarean section during term pregnancy in our hospital were randomly selected. Patients were given morphine intrathecal injection anesthesia. Visual analogue scale (VAS) and comfort rating (BCS) were recorded at 4h, 12h and 24h, respectively. Adverse reactions including nausea, vomiting, respiratory depression and pruritus were recorded within 24h after surgery. Polymerase chain reactionrestricted fragment length polymorphism was used to detect OPRM1 118G gene polymorphism and compare the effects of different genotypes on morphine analgesia and adverse reactions. Results The distribution of wild homozygote, heterozygote and mutant homozygote of OPRM1 allele was in line with HardyWeinberg equilibrium and was representative of the population. VAS scores at 12h and 4h of GG genotype were significantly higher than A/A genotype and A/G genotype（t=14.197，P＜0.05; t=11.502，P＜005; t=16.146，P＜0.05; t=13.269，P＜0.05）. GG genotype BCS score at 12h and BCS score at 24h were significantly lower than A/A genotype（t=2.639，P=0.095；t=2.33，P=0.0214）. The skin pruritus rate of GG genotype was significantly higher than that of A/A genotype and A/G genotype（2=53.152，P＜0.05;2=33512P＜0.05）. Conclusion OPRM1 118G gene polymorphism can affect the analgesic effect and skin pruritus rate after intrathecal morphine injection in cesarean section.