Research Findings: Despite the fact that the universal two-child policy had a significant impact on family structure and parenting behaviors, there is still a paucity of study on the mechanisms underlying this policy's effects. This study explored the role of social competence and sibling presence in the relationship between maternal warmth and prosocial behaviors. Participants were 203 children aged 5-6 years (M[subscript age] = 48.47 months, SD = 3.74;104 boys) and their mothers in China. Maternal warmth was assessed through the observation of mother-child interaction. Child social competence and prosocial behaviors were assessed by teacher. The results showed that:(1) only children had more prosocial behaviors than children with siblings; (2) maternal warmth (T1) was positively correlated with child prosocial behaviors at Time1 and Time2; (3) Social competence (T2) mediated the longitudinal association between maternal warmth (T1) and child prosocial behaviors (T2); (4) The sibling presence moderated the relationship between maternal warmth (T1) and child prosocial behaviors (T2). Specifically, maternal warmth was only associated with subsequent prosocial behaviors for children in one-child families. Practice or Policy: These results revealed the influence of family structure and maternal warmth on child prosocial behaviors especially for children with siblings.