The literature on the effect of private supplementary tutoring has not yet reached a unanimous conclusion. In the context of the marketization of educational resources in China, this paper uses data from the Chinese Education Panel Survey (CEPS) and employs the two-stage least squares method (TSLS) to investigate the causal impact of private supplementary tutoring participation on junior middle school students' academic achievement. After addressing the endogeneity issue of private supplementary tutoring participation, the results show that private supplementary tutoring significantly reduced the overall standardized scores of junior high school students, as well as their standardized scores in Chinese, mathematics, and English. Heterogeneity analysis shows that students attending lower-ranked schools are more adversely affected by private supplementary tutoring. Further analysis shows that the more academic pressure students face, the greater the negative impact of private supplementary tutoring on their academic achievement. Therefore, for students with heavy homework, blindly participating in private supplementary tutoring could exacerbate the academic achievement gap among students, which is detrimental to the balanced development of education. [ABSTRACT FROM AUTHOR]