A method for protecting rotor winding overcurrent of alternating current excitation synchronous motor include ssuch steps as 1) connecting rotor winding three-phase current signal of motor, sampling to obtain instantaneous current value;(2) discriminating the overcurrent of the instantaneous current value to generate a square wave signal representing the instantaneous overcurrent state; 3) calculating that duty cycle of the square wave signal generated in the previous step; (4) counting the protection delay counter: if any wave signal is high or the duty ratio of any wave signal is greater than the threshold, the delay counter is counted accumulatively, and vice versa the delay counter is counted regressively. When the accumulative value of the delay counter is greater than the fixed valueof the delay, the overcurrent protection operation is carried out. The method is based on the instantaneous value of the current for logical discrimination, the protection calculation is small, and there is no inherent delay caused by the phasor algorithm in the conventional protection method, and avoids the error of the fault duration determination, and can quickly and reliably reflect the rotorside short-circuit fault.