To improve the fertility policy system and build a fertility-friendly society, it is crucial to explore the effect of fertility support mix on attitudes toward having a second child. Using qualitative comparative analysis on survey experimental data, this study examines the impact of fertility support mix on the attitudes to have a second child. The findings indicate that, individually, economic support, time support, and caregiving support are positively associated with second-child fertility attitudes. However, the support mixes produce varying synergy effects, either positive or negative. Specifically, a combination of economic support and caregiving support generates positive synergistic effects, amplifying the promotion effects on the attitude towards second-child birth. Conversely, extending maternity leave undermines the positive impact of increased economic and caregiving support on the attitude toward second-child birth among women and highly educated individuals, creating a negative synergistic effect. Furthermore, caregiving support from spouses or parents has a stronger positive influence on attitudes toward a second child birth compared to institutional caregiving support. Based on these findings, the study emphasizes the importance of a comprehensive consideration of synergies among various supports and the critical role of family support in refining the fertility support system.