Abstract: A new underwater system, called the underwater self-reconfigurable system, is presented in this paper. The system consists of a few of kinds but unlimited numbers of modules. By connecting and disconnecting among the modules, its configuration can be changed in real-time. By the modules' metamorphosing consistently, the system can realize different modes of motion. Features of distributed structure and changeable configuration make the system highly flexible for underwater tasks such as observation in complex underwater structure. Development of USS, a prototype of such a system is described in detail. USS now consists of only 1 kind of module, and can realize two 2D configurations and two 3D configurations. Because of the module's capability of buoyancy regulating and one degree metamorphosing, the system can maneuver in 3D ocean space and underwater structure with 2D configurations.