Hydrologic analysis based on the data of remote sensing is a hot topic in the field of hydrology research in recent years.This article extracts the river network based on DEM in southwest area of western Sichuan Yi nationality autonomous region.Through comparing the system with standard river system,it showed that the method is reasonable and the result can be used as the basic information of hydrology research. It analyzed the river network under resolution of both 30 m×30 m and 90 m×90m DEM data,and abstracted river network according to threshold value of ladder distribution.The best river network was extracted. Finally it got the conclusion that extracted density of water network decreased with the increase of threshold and reached the critical value at lower limit 100 and upper limit 50000.