Objectives The purposes of this study was to investigate the effects of young children-mother goodness of fit and mother’s positive emotional expressiveness on young children’s self-assertiveness and examine the mediating effect of maoher’s positive emotional expressiveness on the relationship between young children-mother goodness of fit and young children’s self-assertiveness. Methods A survey was conducted on 212 4,5-year-old children and their mothers, and the collected data was analyzed using the SPSS program. Hierachical regression analysis was conducted to examine the effect of young children-mother goodness of fit on the young children’s self-assertiveness, and the mediating effect analysis of the mother’s positive emotional expressiveness to the effect of young children-mother goodness of fit on the young children’s self-assertiveness was conducted using SPSS Process Macro Model 4. Results First young children-mother goodness of fit had a significant negative effect on the young children’s self-assertiveness, and young children-mother goodness of fit had a significant negative effect on the mother’s positive emotional expressiveness. Second the mother’s positive emotional expressiveness was found to partially mediate the effect of young children-mother goodness of fit on the young children’s self-assertiveness. Conclusions It was found that the higher the young children-mother goodness of fit, the lower the mother’s positive emotional expressiveness, whiich had a negative effect on the young children’s self-assertiveness. Thus, this study was found that there are different circumstances and contexts for interpreting goodness of fit in young children compared to in infant. and it is significant that this study provides foundational material that can activate scholarly and educational discussions regarding the positive emotional expressiveness of mother for the self-assertiveness of young children in future related research and practical parenting contexts.