Abstract: This paper focuses on the design of bionic structure of a robot-fish joint,which depends on a sufficient analysis of anatomical structure of a live fish,by using artificial muscles as its main driver,and introduces a detailed method to design an SMA(shape memory alloy) spring.Meanwhile,the swing model of fish tail fin is established and simplified.Then,based on the torque balance equation of swing motion,the SMA drive torque and resistance torque,flow frictional torque and structure(flow) inertia torque are analyzed,and the quantitative relationship among several parameters is established.This work will provide a significant reference for bionic robot-fish research.