In this paper, 96 soil samples in Beijing farmlands were studied for chemical forms of heavy metals of Cd, Hg, Pb and As by sequential extraction method. The results showed that total concentrations of Cd, Hg, Pb, and As were much lower than the corresponding limits of national standards. As, Hg and Pb were dominated by residual forms, while Cd in exchangeable forms. The variation coefficients of chemical forms of As and Pb were comparatively smaller, whereas, the variation coefficients of each chemical forms of Hg and Cd were comparatively larger. The total concentrations of heavy metals and soil organic matter content were the main factors that affect chemical forms of heavy metals. Cd was comparatively mobile and readily absorbed by plants. The potential hazard of Cd to farmland ecosystem was serious.