Assessing the effect of drought process on the production function objectively is helpful to cultivated land conservation. The effect of drought on production function of cultivated land in Beijing was evaluated. Firstly, the drought conditions were characterized by the surface wetting index (SWI), and the typical drought periods were identified using Mann-Kendall testing method. Then natural potential productivity of non-changed cultivated land was calculated with meteorological data using Thornthwaite Memorial model. Finally, the effect of drought on natural potential productivity of cultivated land was analyzed. Main conclusions were as follows: From 1951 to 2006, the SWI decreased with some fluctuation. The drought was significant during the periods of 1979-1984 and 1998-2006. Annual mean potential productivity was 1 043 g/(m2·a) in 1979, 912 g/(m2·a) in 1984, 1 085 g/(m2·a) in 1998, and 872 g/(m2·a) in 2006. The natural potential productivity in Tongzhou County was bigger than those in other counties, but that in Yanqing County was the smallest. The drought had great impacts on natural potential productivity. The total potential productivity loss exceeded 10% during the period of 1979-1984, and accounted for nearly 20% during the period of 1998-2006. Therefore, the government should simultaneously take into account the quantity and distribution of the cultivated land, and the impacts of climatic fluctuation especially the drought on the production function in the cultivated land management.