The Fish-imitated underwater robot is developed, researched and designed, and a underwater robot that has a kind of new type of underwater propulsion system is introduced. The efficiency of the propulsion, reducing energy consumption, reduce noise and protect the environment could be effectively improved. Utilizing the characteristics of the crank-rocker mechanism, the rocker swinging back and forth was driven by continuous rotation of the motor, in order to implement propel the underwater robot. The crank-rocker mechanism with the fast-returning characteristic was designed, making the swing speed of fin to the front is slightly faster, in order to realize rapid reset. And the swing speed of fin to behind was made a little bit slower, in order to increase the function time of thrust. Using Solidworks to modeling, MATLAB is used to optimize and design the parts of key structure.