①The variation of the mean grain yield per unit area amounts to two thirds of change of the total grian yield,and plays a decisive role. ② Since the 1950s,percentages of wheat and maize yield in the total grain yield have been individually tending to increase,rice to relatively stable,but that of soybean etc. to decrease.③ So far as the principal grain crop yields per unit area are concerned,the yield of single rice harvest is the highest,the increase of wheat yield is the fastest. The relationships between bumper or poor harvest of the mean grain crops per unit area and weather condition suggests that the connections between precipitation of Northwest and Northeast regions,and accumulated tempertatue of North China and good or bad harvest of the grain crops are intimate. The accuracy of this model established by these factors for assessing good or bad harvest of the grain crops is over 85 Percent.