Objective To investigate the effect of postpartum sedentary behavior on weight retention. Methods A total of 540 women who had maternal inspection and delivery in our hospital from January 2013 to June 2014 were enrolled in the study. Weight and sedentary behavior were investigated on the 42nd day and in 3, 6, 9 and 12 months after childbirth. Cases with the 1st-postpartum-year weight 5 kg over the weight before pregnancy were classified into weight-retention group, and others were into non-retention group. Mixed-effect model and multivariate logistic regression analysis were performed to investigate the relationship between postpartum sedentary behavior and weight retention. Results The average weight retention on the 42nd day, and in 3, 6, 9 and 12 months after childbirth were (7.90 ± 5.21) kg, (7.59 ± 5.52) kg, (6.02 ± 5.23) kg, (4.83 ± 4.26) kg and (3.92 ± 4.01) kg respectively. Postpartum weight retention occurred in 36.1% of the puerpera in 1 year after childbirth. The percentage of puerpera who had daily sedentary time < 5 h, 5-7 h and > 7 h in 1 year after childbirth was 22.6%, 39.6% and 37.8% respectively. Results of mixed-effect model showed that there were significant differences in the weight retention among the groups with different sedentary time (P < 0.01). Multivariate logistic regression analysis showed that the way of feeding and daily sedentary time were independent factors affecting postpartum weight retention (P < 0.05). Conclusions Postpartum weight retention situation is not optimistic. Too long postpartum sedentary time could obviously increase the risk of weight retention, so we should strongly advocate the importance of reducing sedentary time and moderate exercise.