The invention discloses a variable stiffness device for a tail drive system of an imitation fish underwater robot. The variable stiffness device for the tail drive system of the imitation fish underwater robot comprises a shell composed of a rudder machine table and a ladder hole structure arranged on the rudder machine table, a rudder machine is arranged on the rudder machine table, the ladder hole structure is composed of a piston hole and a through hole provided with an inner thread, the through hole is connected with an inner cylinder sleeve through the inner thread, the end of the inner cylinder sleeve is connected with an outer cylinder sleeve with a rubber round pipe, and a rubber cap is arranged at the contact portion between the outer cylinder sleeve and the end of the inner cylinder sleeve, a piston with a lug table is arranged in the piston hole, and an elastic retaining ring is arranged between the piston and the inner wall of the piston hole, a cam is arranged on the driving rod of the rudder machine, and the cam contacts with the lug table. According to the variable stiffness device for the tail drive system of the imitation fish underwater robot, the characteristic of elasticity of the rubber round tube is creatively used, the overall stiffness is changed by injecting gel, the achieving method is simple, practical is high and reliability is relatively high, and provides a very good idea for the improvement of a imitation fish underwater robot driving system.