Using ground control points (GCPs) can improve positioning accuracy in the calibration process, but the acquisition of GCPs is always difficult and high cost, especially in mountain areas and abroad. Thus, we proposed an improved calibration algorithm assisted by the digital elevation model data to calibrate geometric parameters for the single-scene synthetic aperture radar (SAR) image with no need for GCPs by providing an alternative solution that introduces topographic slope as an index of applicability and reliability. The algorithm includes three main steps: SAR image simulation, registration parameters calculation between the real and simulated images using a very large search window, and time delay estimation in the range and azimuth directions based on the above registration results. Experiments on Gaofen-3 SAR images of different modes show that the proposed approach provides good performance without any GCPs, reducing the errors of checkpoints from 151.3 to 26.6 m. Besides, the orthoimages further demonstrate the effectiveness of the algorithm in the whole image, especially in mountainous areas where the layover is well compensated. Finally, the applicability of the proposed method is analyzed, proving its great suitability for SAR images with terrain slopes greater than 0.05.