As a large scale of project to protect natural environment in China,the impacts of Grain for Grass(GFG)on the total amount of national grain supply should be paid great attention.Based on the home and abroad researches about this topic,the author makes a quantitative analysis on the impacts of GFG on our national macroscopical food safety and its affects level from the angle of short-term and long-term.Then the paper draws two conclusions.One is GFG has not impaired our national grain supply since it has been carried out from 1999,the other one is the areas of GFG in China should be curtailed gradually for a long-term on the condition of ensuring percapita food consumption.