Objective: To evaluate the efficacy of transvaginal cerclage for twins with cervical dilation or short cervix and to explore the indicated cervical length for transvaginal cerclage. Design: Prospective cohort study. Setting: Two tertiary hospitals in Shanghai, China. Population: A total of 177 twins with asymptomatic cervical dilation or cervical length ≤15 mm between 16 ^0/7 and 25 ^6/7 weeks. Methods: The logistic regression model and generalized estimation equation were used to compare the pregnancy outcomes between no-cerclage group and cerclage group followed by subgroup analysis of different cervical length. NNT and Kaplan‒Meier curves were used to estimate the efficacy of cerclage for twins in different groups. Main Outcome Measures: The primary outcome was gestational age at delivery and the neonatal survival rate within 3 months after birth. The secondary outcomes were the gestational latency from diagnosis to delivery and the risk of preterm birth before 26, 28, 32 and 34 weeks of gestation. Results: Compared with no-cerclage group, the gestational age at delivery (32.09±4.50 vs. 28.29±6.20 weeks, p<0.000) and the gestational latency from diagnosis to delivery (10.86 [7.14,13.86] vs. 3.00 [0.50,10.29] weeks, p<0.000) were longer in the cerclage group. The rate of neonatal survival (86.43% [223/258] vs. 47.92% [46/96], p<0.000) in the cerclage group was significantly higher. In the subgroup of twins with cervical dilation or cervical length <10 mm, twins in the cerclage group had significantly longer gestational age at delivery (31.33±4.63 vs. 23.44±4.25 weeks, p<0.001) and gestational latency from diagnosis to delivery (9.07 [6.29-13.57] vs. 0.43 [0.29-1.71] weeks, p<0.001). For twins with cervical length of 10-15mm, although the gestational latency (12 [9.14-13.86] vs. 9.93 [6.29-12.29] weeks, p=0.037) was significantly longer, there was no difference in gestational age at delivery (33.05±4.16 vs. 32.40±4.33 weeks, p=0.300) or neonatal survival rate (87.72% [100/114] vs. 80.77% [42/52], p=0.238) between the two groups. Conclusion: Cerclage was associated with improved maternal and neonatal outcomes in twins with cervical dilation or cervical length <15 mm. More evidence is needed to confirm the efficacy of transvaginal cerclage for twins with cervical length of 10 - 15 mm.