Abstract: To study the spatial differentiation phenomena of precipitation in composite-mountain system, the relationship between the mountain slope, altitude and the precipitation of Beijiang River Basins during the flood season is analyzed with the method of multi-factor separation, using monthly precipitation data from 1959 to 2000 collected from 40 hydrologic stations. In order to eliminate effects of macro geographical factors, the data heterogeneity caused by different locations of hydrologic stations is resolved by using the geographically weighted regressing (GWR) method. The correlation diagram of the multi-year mean precipitation and the altitude is constructed according to the evaluation from GWR, and it is divided into four sub-areas based on the point scattering. On the basis of the division, the correlation between the precipitation change and altitude is researched. According to the changing rate of precipitation with the altitude, Beijiang River Basins are partitioned, and effects of warm air through the upstream and downstream on precipitation change with altitude are studied.The result shows that the distribution of the multi-year mean precipitation during the flood season in Beijiang River Basins is perpendicular to parallel and it decreases from the south to the north. The rainy area is in the southeast of the basin and the mainstream from Yingde to Qingyuan is a stable rainy center. With the change of altitude, precipitation curve shape of the composite-mountain system can be summarized as the parabolic type. The variation of precipitation with the change of altitude in composite-mountain system is related to the precipitation interception effect of warm air through the upstream and downstream of the ridge. Varying characteristics of the average precipitation during the flood season in Beijiang River Basins demonstrate the obvious regional differentiation, it can be divided into four sub-regions, and there are different maximal precipitation in different sub-areas. From the south to the north, altitudes are 77.3 m, 408.4 m, 353.6 m, 376.9 m and the maximum precipitation are 1566.2 mm, 1467.5 mm, 1295.9 mm, 1151.5 mm, respectively. The precipitation during the flood season in Beijiang River Basins gradually increases from south to north, which means that the annual variation of precipitation presents spatial distribution regularities, bigger in north and smaller in south. This is associated with interception effect of mountains to the airflow and warm air strength. When the warm air is stronger, the interception effect on airflow upstream mountain water is relatively small. When the warm air flow is weak, the interception effect of upstream mountains is relatively strong.