Abstract: The naval ship is a large and complicated system overall， and it demonstrates clear levels of diversity. Hence， the system decomposition with Multidisciplinary Design Optimization （MDO）， according to certain rules， is considered to be the basis of the MDO. In this paper， This paper focuses on a particular type of Autonomous Underwater Vehicle （AUV） that uses the Control Moment Gyros（CMGs） for attitude control. It is noticed that the AUV may have a large attitude angle， and as a result， a proper motion model must be established to avoid the attitude description matrix singularity. To do so， the four-parameter approach is applied to establish the kinematics equation， and the relationship between four-parameter and Euler's angle is then given. When constructing the dynamics equation， the AUV is regarded as a multi-rigid-body system consisting of the AUV itself and CMGs， while Euler's angle is replaced by four-parameter. For validation， the motion model is simulated by the Runge-Kutta method. The results show that the model effectively avoids the singularity.