As an important part of the bionic robotic fish, lightweight design of the frame mechanism was an effective way to improve the efficiency of bionic robotic fish propulsion. The frame and tail swing three-dimensional solid model of virtual bionic robotic fish were constructed on the three-dimensional design platform. Based on flapping wing theory, the mechanical characteristics of the tail fin of bionic robotic fish in motion were analyzed, and the instantaneous load was obtained by numerical simulation method. The caudal fin mechanism was introduced into ADAMS environment for dynamic analysis, and the motion load of the bionic robotic fish frame mechanism was obtained. Using the transient simulation of bionic robot fish frame, the real-time dynamic mechanical characteristics of bionic robotic fish were studied. Based on topology optimization, the key areas of the bionic robotic fish frame are lightweight designed. The results show that the equivalent stress, equivalent strain, total deformation and mass are reduced by 12%, 16%, 5.3% and 10.69% respectively. The lightweight design method of bionic robotic fish frame improves the maneuverability of bionic robotic fish and provides a reliable theoretical basis for the design and research of bionic robotic fish.