The sound velocity of ice is affected by temperature, salinity and density, so it is a unstable value that varies with time and space. The three main factors affecting ice sound velocity are analyzed in this paper, and a practicable sound velocity modeling method is proposed as follows: taking porosity as the best characterization factor of ice sound velocity, analyzing a large amount of experimental data on natural ice to establish the mathematic model of the ice sound velocity about porosity first, then using the existed function relationship of porosity with temperature, salinity and density to put forward the ice sound velocity model expressed by temperature, salinity and density. Based on the proposed method, a complete sound velocity model of freshwater ice is established and a corresponding empirical formula is obtained. The river ice sound velocity experiment is conducted to verify the effectiveness of the freshwater ice's sound velocity model. And based on this model, the vertical velocity profile of freshwater ice is studied in this paper. What we have done in this paper would provide a theoretical basis for using acoustic techniques to monitor the thickness of ice and understand the evolution of cold ice layers and the climate change in the polar region and the high latitudes in northern China.