This paper proposes a modular design method for realizing the torpedo-shape robot fish (ldquoHRF-IIrdquo). By biological inspiration, a tail propulsive module and an up-down motion module upon the robot fish are developed respectively. Considering the design principle and power estimation on each module, a robot prototype is completed to conduct the underwater test. The experimental results state that the robot can swim forward or backward, accelerate or decelerate, turn right or left, or even submerge or ascend like a real fish. Comparison of the novel prototype ldquoHRF-IIrdquo with the former ldquoHRF-Irdquo demonstrates that a satisfactory performance of using modular design method is obtained. It is confirmed that modular design served as a modern design method, makes great sense to instruct the further general design of robot fish.