Underwater manipulator as efficient tools plays important roles in ocean exploitation. Because of its specificity for underwater operations, the structure of an underwater manipulator should be designed compactly with light weight and small size. Structure of a 7-function underwater manipulator is designed here. Except forearm rotation and wrist rotation which use hydraulic motors, other functions are driven by hydraulic cylinders, whose parameters are determined by force analysis and limit positions. The hydraulic pipelines from the elbow joint to the end are passed through the forearm, and the hydraulic cylinder of the jaw is integrated into the wrist motor. Built-in pipelines are achieved in the designed structure and the whole weight and volume are greatly reduced by integrated and optimized design.