Shadow education helps students become academically competitive. Currently, little is known about whether shadow education can effectively fulfill either remedial or enrichment purposes in China. Using the nationally representative data from the China Education Panel Survey (CEPS) collected in 2013 and 2014, this paper explored the opportunity gap in shadow education and its impact on Chinese eighth-grade students' mathematics and English reading performance. This study further examined how this effect differs by students' prior performance and school ranks by hierarchical linear modeling (HLM). The results show that affluent socioeconomic resources and parental pressure predicted larger chances of students using shadow education. On average, supplementary tutoring had a positive effect on academic achievement. After re-estimating the effect by groups, this study found that students disadvantaged by prior knowledge and quality of schooling benefited more from supplementary tutoring. The findings suggest that the expansion of shadow education may contribute to social reproduction within the current education system.