<p>This study empirically investigated the mechanisms underlying work-to-family conflict and family-to-work conflict’s effect on second-child fertility intention by focusing on parental burnout and grandparent support. A sample of 1072 Chinese respondents who had already had one child were surveyed. The results of the two-wave longitudinal study showed that both work-to-family conflict and family-to-work conflict exerted a significant negative effect on second-child fertility intention. Mediation path analysis revealed that both independent variables had significant positive effects on parental burnout, while parental burnout exhibited significant negative effects on the dependent variable. Subsequently, our study confirmed that parental burnout served as a mediating variable between work-to-family conflict, family-to-work conflict, and second-child fertility intention. Furthermore, grandparent support moderated only the direct positive effect of work-to-family conflict on parental burnout and the indirect effect of work-to-family conflict on second-child fertility intention via parental burnout. Thus, the indirect effect of work-to-family conflict on second-child fertility intention is weakened when grandparent support is high. The study offers implications that mitigating the negative effects of family-to-work conflict on second-child fertility intention require support not only from the micro-ecosystem of the family, but also from the meso- and macro-ecosystems, such as organizations and society.</p>