This study aimed to examine the association between parent-grandparent co-parenting and children's self-control as well as the potential moderated mediation mechanisms. Participants were 367 children aged from 3 to 6 years from two public kindergartens in Shanghai and Guangzhou, China. Their mothers were asked to complete multiple questionnaires including the parent-grandparent co-parenting scale, parenting style scale, and children's self-control, as well as their grandparents were asked to rate parenting practices. Research Findings: The results showed that: (1) parent-grandparent co-parenting was positively correlated with maternal authoritative parenting, grandparental authoritative parenting, and children's self-control, whereas negatively correlated with maternal authoritarian parenting and grandparental authoritarian parenting. (2) maternal authoritarian parenting mediated the relation between parent-grandparent co-parenting and children's self-control; (3) grandparental authoritarian parenting moderated the relations between parent-grandparent co-parenting, maternal authoritative parenting, and children's self-control. To be specific, for children whose grandparents adopted less authoritarian parenting, their self-control level was higher, and the parent-grandparent co-parenting could predict their self-control via maternal authoritative parenting. Practice or Policy: These findings provide valuable information on the importance of maternal and grandparental parenting styles between parent-grandparent co-parenting and preschoolers' self-control in Chinese culture.