In order to improved the efficiency of induction motor (IM), on the basis of considering the iron loss mathematical model, voltage and current constraints at high and low speed conditions were given. The given of the optimal rotor flux linkage to ensure the smallest loss was transformed into the minimization of the objective function under constraint conditions. The generalized Lagrange function was defined, and the Kuhn-Tucker theorem was used to solve the optimal rotor flux linkage under constant torque region and constant power region, then IM efficiency was maximized in the entire working range. Simulation results verified the effectiveness of the proposed method.