Abstract: Based on the principle of water volume balance and kinematic-wave (KW) model , those models are disposed with dimensionless, which to improve simulation accuracy of water movement process with water volume balance model. The changes of surface shape factor were studied and it could be computed with empirical formula which was proposed in this paper. Research results showed that the surface shape factor was not a constant value, but a variable value with the change of irrigation time, water advance distance and other factors. The validity of the proposed method was verified by some research materials, the results showed that the precision of the model could be improved when the variation of surface shape factor was considered using volume balance model and the empirical formula proposed. The model was applied to estimate water surface shape factor for forder irrigation was reliable.