Timely intervention is essential to build resilience and foster positive emotional experiences as mothers navigate the challenges of motherhood. To support specific interventions, this study examined the mediating effect of maternal role adaptation on the relationship between perceived social support and new mothers' positive affect, as well as the moderating effect of the postpartum months in this relationship. The predictive effect of perceived social support on positive affect in the postpartum period remains unknown, particularly within specific time windows. To clarify this, mothers from the outpatient department of a large public children's hospital in Shanghai, China, were surveyed in June and July 2019. A total of 498 mothers who had given birth in the last year completed the survey. They reported postpartum months and sociodemographic information and completed the maternal social support scale, the positive and negative affect scale, and the maternal role adaptation scale. The results indicated that social support positively correlated with maternal role adaptation and positive affect. Maternal role adaptation was a mediator between social support and positive affect, after controlling age, years of marriage, employment status, household income, education, and birth number. Postpartum months moderated the mediating effect of maternal role adaptation on positive affect. This effect was only for mothers with babies six months old or less. The results point to specific time windows for the predictive role of social support in improving positive affect. Clinical interventions to improve positive affect during postpartum should pay attention to the "golden period" before the baby's sixth month.