This paper addresses the mechanism design of a novel lightweight underwater manipulator. This underwater manipulator has the properties of light weight, small size and uniform mass distribution. Specifically, the implement of every joint and the gripper of the underwater manipulator is presented. Then the forward kinematics model is built based on Denavit-Hartenberg principle, and the inverse differential kinematics model is established by virtue of Jacobian relationship. Moreover, the forward and backward recursive dynamics propagation model is introduced with consideration of hydrodynamics. Finally the simulation is conducted to verify the practicability of the underwater manipulator.