<p>Developing countries are currently facing severe challenges such as persistent poverty and gender imbalances. However, existing literature on the relationship between the sex composition of children and vulnerability to poverty remains limited within a unified analytical framework, particularly in Asian contexts where cultural traditions emphasize the impact of a child’s gender on family outcomes. In this study, we use the data from the China Family Panel Studies covering 2016 and 2018 to explore the relationship between the sex composition of children and vulnerability to poverty. We found three noteworthy findings: (1) Chinese households with a higher proportion of sons strategically reduce vulnerability to poverty, which we propose as “competitive poverty prevention”. (2) Such effect is more significant in families with children aged 21–40 or over 40 years than those with children aged 16–20 years. (3) Mechanism analysis revealed the competitive prevention pathways for reducing vulnerability to poverty is driven by the enhancement of parents’ endogenous motivation. In the context of gender imbalance, Chinese parents with sons exhibit stronger endogenous motivation to increase household savings and accumulate assets to meet an advanced competitiveness of their sons in the marriage market, effectively reducing vulnerability to poverty.</p>