This study investigated the effects of grandparents’ involvement on preschool children’s social competence and its underlying mechanism. Six hundred and ninety-one mothers of 3-6 years old children were asked to report their parenting stress, parenting self-efficacy, their children’s social competence and grandparents’ involvement through online questionnaires. The results showed that:1) Grandparents’ involvement and children’s social competence were significantly positively correlated. 2) Maternal parenting stress and maternal parenting self-efficacy played partial mediating roles between grandparents’ involvement and children’s social competence. 3) Maternal parenting stress and maternal parenting self-efficacy played significant chain mediating roles between grandparents’ involvement and children’s social competence.