A dynamic uniform Cartesian grid system was developed in order to reduce the computational time in inundation simulation using a Godunov-type finite volume scheme.The reduction is achieved by excluding redundant dry cells,which cannot be effectively avoided with a conventional Cartesian uniform grid system,as the wet area is unknown before computation.The new grid system expands dynamically with wetting, through addition of new cells according to moving wet-dry fronts. The new grid system is straightforward in implementation. Its application in a field-scale flood simulation shows that the new grid system is able to produce the same results as the conventional grid,but the computational efficiency is fairly improved.