Accurate measurement of underwater terrain in coastal areas is the key to human activities in the ocean. Synthetic aperture radar (SAR) provides a new means for the detection of shallow sea terrain. TerraSAR-X bunching mode can obtain high-resolution SAR data with a long integration time, so that the seabed terrain of shallow sea can be inverted more accurately. Traditional SAR image underwater terrain detection is based on the assumption of constant wave period, which not only needs to know the initial water depth to solve the wave period, but also brings some errors to the detection results. In this paper, an underwater terrain detection method based on subaperture images is proposed. A SAR image is decomposed into subaperture images with fixed multi-scene time intervals, and the time interval between subaperture images is used to solve the changing wave period, so as to obtain more accurate underwater terrain. The TerraSAR-X data of Wuzhizhou Island in Hainan Province was used to verify the feasibility of this method. The results obtained by inversion of this method were compared with the GEBCO data, and it was found that the two were in good agreement（the MAE was 2.8m and the MRE was 23.91%）. It is proved that this method has great potential for retrieving water depth in shallow sea.