According to the data base of Chinese environment resources,on the basis of the reearches on the changes of the Chinese farmland area in 10 years,the gravity centers of the farmland areas of two periods are calculated by using the method of the gravity center model. From this it is found that the gravity center of the Chinese farmland has moved 28 337 7km toward the northwest in 10 years. Then,on the basis of the fuzzy evaluation of the farmland ecological background,the quality index of the farmland ecological background is designed. By using this index the quantitative value of the decline of the Chinese farmland quality is calculated. The value is 2.52%,which is induced by the movement of the gravity center of the farmland.