This article selects 8 main factors (the number of rural employees, total power of agricultural machinery, effective irrigation area of crops, growing area of grain crops, fertilizer consumption, electricity consumption in rural areas, area affected and area covered) as the factors influencing grain output, and offers the method of determining weight of factors influencing grain output using entropy weight method. According to the relevant data in the period 1985-2005, we analyze the weight of factors influencing grain output in China by example. The results show that the electricity consumption in rural areas has the greatest impact on grain output, followed by total power of agricultural machinery, fertilizer consumption and area covered. To increase grain output, we must enhance the degree of mechanization, free people from the former process of direct cultivation, strengthen water conservancy construction, and do a good job in disaster prevention and mitigation.