Research Findings: The impact of the newly launched "two-child policy" on young children's social development has not been empirically studied. This study examined the possible mediating and moderating factors in the relationship between parenting styles and social competence in Chinese preschoolers. A sample of only children (N[subscript only-child] only-child = 352, M[subscript age] = 3.96, SD = 0.62,) and children with siblings (N[subscript child-with-siblings] = 552, M[subscript age] = 4.02, SD = 0.64) was recruited from Fuzhou, China. Their parents completed the Children's Behavior Questionnaire (CBQ), Social Competence and Behavior Evaluation (SCBE), and Parenting Style and Dimension Questionnaire (PSDQ). The results indicated that: (1) the only children were more likely to have parents of authoritative and permissive parenting styles than the children with siblings; (2) the children with siblings had higher levels of self-regulation than the only children; (3) self-regulation mediated the relations between all dimensions of parenting styles and children's social competence; (4) singleton (only children or not) moderated the mediating effect of self-regulation in the relationships between permissive parenting style and children's social competence after controlling for child age and gender. Practices and Policy: The results have demonstrated the influence of the "two-child policy" and how such macro-contextual change interacts with key microsystems such as the family and parenting behaviors in relation to early development. The findings have several practical implications for parent education and home intervention.