This article investigates determinants and impacts of supplementary education for primary-school students in Beijing, China. The economic condition is an evident determinant of external supplementary education but insignificant for internal supplementary education. Supplementary education can significantly improve academic performance, physical health, and non-cognitive development. The impact of external academic supplementary education on academic performance is larger than that of internal one, while impacts of internal and external non-academic supplementary education on physical health and non-cognitive development are almost identical. The government should substitute external supplementary education with internal one or provide subsidies for disadvantaged students to maintain education equality.