Abstract: The cut-off negative pressure method is one of effective approaches to solve the seepage flow problem with free surface. According to the characteristics of cut-off negative pressure method, however, it is found out that the deficiency of the algorithm is to easily cause inexact simulation in the vicinity area around free surface owing to the penalty function with only one controlling penalty parameter. Therefore, the improved cut-off pressure method is presented by constructing penalty function with two controlling penalty parameters. The improved algorithm is able to improve the simulation precision and convergence speed through considering the effect of the partially saturated or unsaturated elements more precisely and calculating the node forces more accurately. Finally, the improved cut-off negative pressure method is applied to simulation of unsteady seepage flow with free surface, and the corresponding program codes are developed with validation of some examples.