From the morphological characteristics of crop growth, the mechanism model of photosynthate distribution based on the dynamic ralationship between root, shoot, leaf and grain is proposed in this paper. And the method of the determining model parameters with the field experiment data is proposed. The root-cap growth equilibrium coefficient, the shoot-leaf growth equilibrium coefficient, the grain-shoot growth equilibrium coefficient, the water stressed coefficient and the nutrient stressed coefficient are obtained by the nonlinear program according to the measured dry matter of the winter wheat root, shoot, leaf and grain in 2008 in the irrigation experiment station of Tianjin argricultutal university and the objective function of the minimum absolute error sum on the simulated and measured dry weight. And they are verified by the the winter wheat measured data in the irrigation experiment station of Tianjin argricultutal university in 2009. The results show that the simulated value and observed data of the dry weight of root, stem, leaf and grain are consistent. The correlation coefficients (R2) of the comparison between simulated and measured value of the root, shoot, leaf and grain dry weight on every treatments were all over 0.81. It shows that the model on the simulation of the root, shoot, leaf and grain in this paper is reasonable. It provides a theoretical basis on reasonable farmland irrigation and fertilization.