The effect of phosphorus application rate and its interaction with soil moisture or/and temperature on the self-diffusion coefficient (Da) of phosphate in four textural unsaturated soils was studied. Although a linear increase in the Da value with increasing phosphorus application rate from 0 to 400 mg per kilogram soil was observed in all the soils, the increase extent was greater in the lighter textural soil or the lower buffer capacity soil at the same phosphate rate applied. The Da values influenced by the interactions of factors such as soil moisture, temperature and phosphorus fertilizer rate applied in different soils were different. In order to express the difference in quantity, according to the nonlinear multiple regression equations between Da value and the factors, the "water coefficient", "temperature coefficient" and "fertilizer coefficient" of phosphate diffusion rate were proposed in this paper using the partial differentials of the equations. It was found under the same conditions, these coefficients in the four soils decreased in order as loessal soil>black lu soil> lou soil>yellow cinnamon soil, which.indicated that the effects of the interactions between the factors on the Da value were closely related to soil texture.