The invention provides a method for determining an optimal time constant of a three-phase asynchronous motor rotor. The method includes the step s11 of setting a rotor time constant reference value tau[r]_ref, the step s12 of selecting an excitation current value id and a corresponding torque current value iq, wherein at the moment, iq=id; the step s13 of calculating the slip ratio Ws=iq / (tau[r]_ref*id) at the moment, the step s14 of conducting vector control, the step s15 of judging whether the torque progressively increases or not, the step s16 of enabling tau[r]_ref to be equal to tau[r]_ref plus delta tau[r] if the torque progressively increases, and executing the step s13 again, wherein delta tau[r] is the increment; the step s17 of determining the optimal time constant tau[r] which is equal to tau[r]_ref if the torque does not progressively increase. According to the method, the defects in the prior art are overcome, the executing efficiency of software is improved, the characteristics of parameter dynamic changes is also taken into consideration, and the method can be easily achieved in terms of projects under the condition that efficient output of a system is ensured.