Abstract: How to process depression and flat areas and their drainage direction is a key issue and must be solved prior to the extraction of drainage network based on the digital elevation model(DEM).This paper investigates the shortcomings of the existing methods for the complex terrain DEM with depressions and flat areas,and proposes several new methods,namely,the classification and the merger of depressions,the valid filing depression,the classification of flat areas,and the construction of the drainage direction based on the flow cost for the river valley plains.These methods can be performed in the software system developed by authors. The test shows that the methods mentioned above can be used to process complex depressions validly and construct flat areas water flow direction in the consideration of the ambient terrain and astringency to the potential waterway. And they are proved to be rationa. In addition,by using the methods,odd waterways such as parallel and pseudo ones can't be generated in the extraction of the drainage network. Hence the results obtained are consistent with the real situation.