Hydrological analysis of precipitation-runoff routing process in watershed is of very importance. Based on the grid system in GIS, the precipitation-runoff routing was numerically simulated with the computer through a series of related data sets making use of DEM's function to provide topographic features. It makes overland flow in the spatial range of a watershed be traced with the correct hydrological sequence and the hydrological computation rank of the runoff path in the watershed be determined of a spatial range. This provides spatial analysis basis for the modeling research based on the precipitation-runoff relation in the watershed. Finally the research achievement was applied in the Huangjiaercha small watershed, the simulation technique was verified to have a definite reliability and practicability.