Abstract: This paper analyzes the reason of systematic error forming, the sound velocity error and attitude error are thought to be two main contribution factors. Although the error character themselves shows random, their effect for depth presents systematic. For a sonar system, according to the nominal accuracy, in whole systematic error, the contribution of sound velocity error for depth is about 35%, and the contribution of attitude error for depth is about 65%. Based on the proportion, according to the character of non-parametric (semi-parametric) method, the systematic error in depth is estimated and weakened.