Abstract: Cultivated land is the base of agriculture production, and is an important factor to ensure food security. Through analyzing cultivated land quantity change and its impacts on grain production, the authors identify that: (1) The total area of cultivated land and the per capita area of cultivated land have decreased continuously from 1978 to 2004, the per capita area of cultivated land decreased to 0.055 hm2 in 2004. (2) The total grain yield took on increasing trend from 1978 to 1999, and it came to maximal level in 1999. Since 2000 the total grain yield decreased a lot. (3) Cultivated land quantity change influenced the total grain yield at all times. Cultivated land quantity change had better correlation to the total grain yield from 1979 to 1982. Since 1983 cultivated land quantity change had lesser correlation to the total grain yield, it indicated that cultivated land quantity change had lesser impacts on the total grain yield. But, due to the restriction of marginal utility descending law, the increase of the total grain yield impossibly absolutely depends on the increase of grain yield of unit area, and keeping the quantity balance of cultivated land is the precondition of ensuring food security.