In this study the two dimensional hydrodynamic model to simulate the propagation of flood shock on the steep mountainous area have been developed. This model is formulated by a hydrodynamic system with the assumption of unsteady flow as steady flow which can reduce computation time. The numerical computation for this model is executed by use of the eight point implicit scheme. The Incremental Dynamic Programming (IDP) technique is adopted for the optimization of grid size during the computation. The model is applied to compute the propagation of flood shock on the steep mountainous area located in southern part of the Korea. Comparison of the results between the proposed assumption model and the general type of two dimensional model indicates the proposed model is nearly as good as, and computationally much faster, than the general two dimensional.