Underwater project is a new kind of arm developed for soldiers under water with vary depth . The effect of the different depth on the shooting situation should be considered efficiently when designing underwater project. It is found during the test that with water depth increasing, the maximum pressure in the bore will increase while the initial velocity of underwater project will decrease accordingly, from which an idea that the effect of water depth can be converted into second work coefficient and a method for calculating different water depth in interior ballistic are raised. With the experiment data testifying the feasibility as well as creativity of this method has offered a theoretical instruction to the research.