Abstract: This paper is used for predicting "CR-02" AUV unpowered diving before the sea test. It's based on developing mathematical model for AUV unpowered diving motion, theoretical analysis of steady motion of AUV, presents the theoretical expression about diving pitch θ and diving depth velocity u_z,those are important parameters used to describe diving motion and can be calculated quickly. Appropriate diving ballast can increase AUV diving speed and reduce diving time. This method has practical value.