This paper presents a method to produce DEM of steep mountains from InSAR interferogram without using any GCP. The SRTM data of the same area are used to assist the conversion from unwrapped phase to height. It has been proved that the main error sources of InSAR DEM generation are interferential phase and baseline estimation. The systemic part of these errors can be removed by some ground control points (GCPs). However in wilderness it's too difficult to build and identify any GCP. Since 3 arc seconds resolution SRTM data of the most part of the world have been in public by now, instead of GCPs, we improve the quality of InSAR DEM in mountainous area by using SRTM data. In this method, certain points with higher coherence and lower phase slope are selected first from the unwrapped interferogram. An iterative process is used on these points for calculating the phase difference in slant range space between InSAR and SRTM data. The points converged in certain stable location are selected finally. Through these points the phase trends is removed by linear regression and the baseline in the whole image is modeled. Then, the different height between InSAR and SRTM data are calculated pixel by pixel, and these height differences are added to SRTM data to get each InSAR pixel's height, latitude and longitude in WGS84. Experiment result using ERS-1/2 data, which is compared with authoritative 1:50000 DEM over Three Gorges, China, is given.