The first-born child's quality may be affected by a younger sibling in a family based on the quantity-quality trade-off theory. Using data from the China Health and Nutrition Survey, we examine the causal effect of having a younger sibling on the health of the first-born child aged 2-12 in China. We use instrumental variables to address the potential endogeneity of having a younger sibling in the extended regression model. We found that having a sibling significantly decreases the height-for-age z-scores of the first-born child, and the greater age gap may alleviate the effect. Further analysis shows that the effect is particularly strong for the preschool child under 6 years old and the child in a low-income family or the rural area. A sibling influences the first-born child's health by dietary pattern, physical activities, and medical services utilization. The robustness checks, based on individual fixedeffects model and propensity score matching approach, validate our findings, which suggest that future preventive intervention on the deterioration of first-born child's health during the implementation of the universal two-child policy.