This study examined the differences between the one-child family and two-child family regarding the effect of social support on parenting stress through parenting self-efficacy using multiple group structural equation modelling. A Chinese sample of 6131 one-child families and 4816 two-child families participated in the study. Parents independently completed self-report measures to evaluate social support, parenting self-efficacy and parenting stress. The results showed that the two-child family reported more parenting stress, less social support and lower parenting self-efficacy than the one-child family. Social support directly and indirectly alleviated parenting stress for the one-child family, and it only indirectly related to parenting stress through parenting self-efficacy for the two-child family. Overall, this study emphasized the effect of social support and parenting self-efficacy on parenting stress, and provided an empirical basis for alleviating parenting stress for parents with one or two young children.