Background Despite the relaxation of fertility restrictions, China’s birth rate continues to decline. The Universal Two-Child Policy encourages couples to consider having a second child, often leading to a bargaining process between spouses with differing preferences. Additionally, the skewed sex ratio has increased Chinese women’s bargaining power, highlighting the importance of analyzing fertility decisions through marital bargaining. Methods This paper investigates second-child fertility decisions using data from the 2018 China Family Panel Studies and employs Ordinary Least Squares regression. The study examines assortative matching based on fertility preferences and uses the 2020 provincial-level sex ratio for individuals aged 20–39 as a proxy for women’s bargaining power in the marriage market. Results The study shows that achieving consensus on having a second child requires cooperation between spouses, particularly when their fertility preferences differ. The study also reveals that marriage matching is not random; individuals are more likely to partner with those who share the same second-child preference. Additionally, women with greater bargaining power positively influence their husbands’ desired family size, a correlation not observed in males. Conclusion The study concludes that second-child fertility decisions in China are significantly influenced by marital bargaining and the increased bargaining power of women due to the skewed sex ratio. Cooperation between spouses with differing fertility preferences is crucial for reaching a consensus on having a second child.