A modular autonomous underwater vehicle is introduced, which has basic navigation ability and can realize other functions through extending. 3D parametric design software Solidworks and computational fluid dynamics software CFX are used to analyze and optimize the shape, which can reduce the power consumption. Modular design is adopted in every function module and the connection form between two sections, which simplifies the structure and makes maintenance easy. In addition, the theoretical calculation and software simulation are performed on the strength and stability of pressure hull. Lake tests prove that the autonomous underwater vehicle can complete basic navigation functions with their own sensors, such as independent constant-orientation sailing, independent constant-speed sailing, independent depth-keeping sailing and position closed-loop sailing.