Based on the difference of permittivity between water and rock media, the authors conducted forward simulation of two-floor geology at different depths of water by GPRSIM software. Under normal circumstances, when the radar antenna frequency is 50MHz, the exploration depth is about 10m. In combination with the geological data of the bridge site area of the Nanjing Yangtze River Bridge, the authors established the underwater sand model which contains water, silty clay, silty sand, gravel pebble and conglomerate, and made forward simulation of the model based on GPRSIM software. The result shows that, when the antenna frequency is 50MHz, the maximum depth of water is less than 4m, and each rock formation under the water has a good reflection. In particular, the geological radar can better reflect the thickness and distribution range of the underwater sand. The results achieved by the authors provide important theoretical basis for the actual exploration of the underwater sand.