Abstract: Multi-beam echo sounder(MBES) is the best equipment for underwater topographic survey nowadays. This kind of equipment can accurately determine the position of sounding point relative to transducer by measuring the round-trip time and angle of acoustic wave. However, due to the change of temperature and salinity in the water, the sound velocity varies with depth, which leads to the sound wave does not travel along a straight line in the water. The refraction of sound wave in the water conforms to Snell's law. In order to accurately determine the sound propagation track and obtain the precise coordinate of sounding points, constant-gradient acoustic line tracking algorithm is most commonly used. however, constant-gradient ray-tracking algorithm often suffers from complicated processing and low efficiency when facing massive bathymetric data, especially in deep sea.Therefore, a sound ray tracking algorithm based on template-interpolation of constant-gradient sound velocity is proposed in this paper to improve the post-procesing efficiency of MBES bathymetric data. Firstly, the principle of our proposed algorithm is studied and the conclusion that the beam footprint can be determined by the template interpolation is verified. Then, the proposed algorithm in this paper, which mainly includes the construction of constant-gradient sound ray tracking template, over-tracking and under-tracking algorithms, draft influence and compensation models, is used in the experiments of shallow-water, middle-water and deep-water measurements. The experimental results show that our proposed algorithm can greatly improve computational efficiency while ensuring tracking accuracy.