Abstract: This paper presents the principle of cosine backscatter model. The model has two unknown numbers: azimuth angle and range angle. For flat area, we assume the azimuth angle is zero, so we can get range angle. Then we can get DEM by integration. Based on this model, we introduce a new algorithm that does not omit azimuth angle and can get DEM in mountainous area. Using this model, we assessed the DEM of Zhangbei of Hebei province.