According to the path modle of causality within the compositive factors of wheat yield,the main factor could be decided through calculating and comparing the total of effect and the total decided level of every factor to yield.On the condition, higher level of the soil fertility with plenty water and fertilizer, and wheat plangted after rice by rotary tillage and broadcast sowing,the main factor was kernel number per spike.On the condition of less water and fertilizer, the main factor was kernel weight. On the condition of sandy and leaking soil, the main factor was spike number.