Abstract: In the traditional concept of distributed hydrological models, calculation of the spatial distribution of grid soil capacity has always been a difficult problem. The value of average storage capacity acting as a key parameter has been considered in the traditional Xinanjiang model. Due to the fact that both Xinanjiang model and topography-based hydrological model (TOPMODEL) use similar theories in their definitions of soil moisture deficit, in this paper the relation between grid storage capacity and the maximum thickness of unsaturated zone is established by the relevant theory in the TOPMODEL, then a distributed hydrological simulation system based on natural sub-catchment is obtained. Application of this system to the Xinanjiang catchment in Guangdong Province shows that its method is effective. Relative to traditional parameter calibration, this method, based on the laws of physics, can calculate soil storage capacity at specific spatial scales with higher accuracy. Thus, it lays a basis for establishment of a distributed Xinanjiang model.