The invention discloses a variable impedance control method for fluid-driving series-parallel-connection biomimetic fishes, which solves a problem of non-viscous proportional damping decoupling problem caused by water environment dynamics. The variable impedance control method comprises steps of 1, constructing a modal decoupling matrix T, 2, performing modal coordinate transformation, 3, applying a modal space impedance control law, and 4, regulating a proportional gain maintrix Kp and an impedance gain coefficient Kf of a mode controller of each order. The variable impedance control method utilizes a fluid-driving series-parallel-connection mechanism characteristic to construct an impedance control method for regulating a mode impedance in a modal space, can effectively eliminate a overdamping characteristic caused by the water environment, stimulates modal resonance of bionic fishes, and finally realizes a bionic fish swimming mechanism that the bionic fish utilizes the modal resonance to perform swinging and propelling. The variable impedance control method is clear in a physic meaning, and simple and effective.