A Scotch yoke mechanism can provide high thrust by generating high frequency flapping motion in a propulsion system of robotic fish. However, higher frequency makes higher drag force, and if the motor torque is insufficient compared to the drag force, this causes a lagging phenomenon due to the lack of torque, which causes a problem of lowering the thrust. In this paper, we propose an efficiency enhanced scotch yoke mechanism through output torque re-distribution. We focus on the fact that the required motor torque of the Scotch yoke mechanism is not constant in the fin propulsion system; thus, it is not necessary to raise the torque in the entire domain. By transferring the extra torque in a certain section to the torque-insufficient section, the frequency and thrust can be increased without increasing the motor size and energy consumption. We present a dynamic model and a design method of the proposed concept, and also validate the performance enhancement by comparing the thrust, frequency, and the period of the tail fin movement of conventional and enhanced system while the equal input current is supplied.