This study aimed to examine the influence of parent-grandparent co-parenting on children's problem behaviors and its potential mechanisms. Participants were 412 children aged 3 to 6 years from four kindergartens in Shanghai, China. Their mothers were asked to complete a series of questionnaires including the parent-grandparent co-parenting scale, parenting style scale, parent-child relationship scale, and their grandparents were asked to complete children's behavior evaluation scale. Research Findings: The results showed that--(1) Supportive co-parenting was associated with less internalizing and externalizing problem behaviors in preschool children, and unsupportive co-parenting was associated with more internalizing and externalizing problem behaviors; (2) Supportive parent-grandparent co-parenting was negatively associated with children's internalizing and externalizing problem behaviors mainly through maternal authoritative parenting and mother-child conflict, and unsupportive parent-grandparent co-parenting was positively associated with children's internalizing and externalizing problem behaviors mainly through maternal authoritarian parenting and mother-child conflict, or directly through mother-child conflict. Practice or Policy: These findings provide valuable information on the importance of parenting styles and parent-child relationships between parent-grandparent co-parenting and children's problem behaviors in Chinese culture.