The Quantity-Quality trade-off model predicts a negative relationship between the number of children and the quality of human capital investment per child. Given that a family has limited resources, having many children makes it more difficult for parents to make high-quality human capital investments for each child. This study explores the empirical relevance of the Quantity-Quality trade-off in China, by examining the effects of China’s family planning policy change on the first child’s human capital investment and parent-child relationship. Our identification strategy utilizes the “selective two-child” policy in China, which suddenly and unexpectedly relaxed the severity of family planning for many Chinese parents who were only children themselves. Our estimation results, based on four waves of panel data from the Chinese Family Panel Studies, are in line with the Quantity-Quality trade-off model. We find that the new family planning policy increased the probability of having a second child and significantly influenced the first child’s educational outcomes and parent-child relationship.