In general, private supplementary tutoring is considered an effective means of improving academic achievement by parents. However, previous studies have produced partial or inconclusive results regarding its effectiveness. Thus, the present study conducted a comprehensive analysis based on a specially designed longitudinal survey of private supplementary mathematics tutoring among middle school students in China. The hierarchical linear analysis did not show a considerably significant positive effect of regular private tutoring, even when such tutoring continued throughout the summer vacation and school semester. The data also allowed a deeper inquiry into the role of the quality, quantity, and applicability of private tutoring. In addition, the heterogeneous effect of private supplementary tutoring across students with different mathematics achievement levels was explored, with possible explanations including the different learning styles of students from different groups, the ceiling effect of the testing tool, and the different learning needs of different groups of students. These results suggest that parents should carefully select private tutoring for their children, and that the government should provide more comprehensive professional guidelines to regulate the industry.