Private supplementary tutoring (PT) has expanded significantly and attracted widespread public interest in China in recent years. It deserves systematic empirical research and reflection, given its crucial theoretical and practical implications. Based on the data of a large-scale survey and testing project, this study examined its effects on the students' academic achievements in mathematics. The results revealed that PT in the current semester did not have any significant positive effect on the students' academic achievement in mathematics. This suggested that households' huge financial investment in PT failed to achieve the desired result. Accordingly, these findings imply that parents should carefully select private tutoring for their children, and the government should provide more comprehensive professional guidelines to regulate the private tutoring industry.