In the general application of bathymetric sidescan sonar, the amplitude information or phase information of the echo data is used alone to obtain a sidescan map or bathymetric map to show the seafloor detailed features. In order to extract the microgeomorphic information inside the sidescan data and realize a higherprecision seafloor terrain detection, a twostep loop iterative algorithm is proposed. Firstly, the primeval bathymetric data and sidescan data are used to optimally fit the scattering model. Secondly, the brightness error correction factor is introduced to the improved shape from shading method and iterate the terrain, which ensures fast and stable convergence in terrain iteration. Finally, through loop iteration, the seafloor sediment parameters and the terrain depth value that has higher precision and stronger correlation coefficient with the real terrain relief are obtained. Meanwhile, the Jackson seafloor scattering model is used to simulate the signal transmission and reception processes of the bathymetric sidescan sonar, and the echo data are used to verify the correctness and effectiveness of the iterative algorithm described in this paper. The results show that the proposed method can effectively correct the terrain, and the higher the received signaltonoise ratio is, the better the terrain correction effect will be. When the signaltonoise ratio is to 20 dB, compared with the original bathymetric result, the corrected correlative coefficient of terrain relief is elevated by 524% and the absolute value of the terrain error is reduced by 37%. At last, the algorithm is applied to the bathymetric sidescan sonar data. The comparison and analysis of the terrain maps before and after correction verifies the feasibility and effectiveness of the proposed algorithm.