This study examined whether parenting styles mediated the relationship between unsupportive intergenerational co-parenting and children’s social competence, and whether the first stage of the mediating process, as well as the direct association between unsupportive intergenerational co-parenting and children’s social competence, was moderated by maternal psychological flexibility. The theoretical model was tested using data collected from 412 mothers of children aged 3–6 years at four kindergartens in Shanghai, China. The results showed that: (1) unsupportive intergenerational co-parenting was negatively associated with children’s social competence through decreased maternal authoritative parenting and increased authoritarian parenting and (2) the first stage of the mediation mechanism was moderated by maternal psychological flexibility. Specifically, unsupportive intergenerational co-parenting was significantly associated with authoritative and authoritarian parenting for mothers with low and high psychological flexibility, respectively, and the magnitude of the association was higher for mothers with low psychological flexibility. These findings extend the understanding of how and when unsupportive intergenerational co-parenting impacts children’s social competence.