The invention discloses an optical transformer-based internal asymmetric fault protection method for a generator. Three-phase currents of a generator terminal side of the generator and three-phase currents of a neutral point side are measured by an optical transformer respectively; a protection device calculates the following parameters: (1) a real part, a virtual part and an effective value of a negative-sequence current of a generator terminal; and (2) the real part, the virtual part and the effective value of the negative-sequence current of a neutral point respectively; and an internal asymmetric fault of the generator is judged and protection acts quickly than tripping when the three conditions are simultaneously met as follows: (1) the negative-sequence current of the generator terminal is greater than a constant value; (2) the negative-sequence current of the neutral point is greater than the constant value; and (3) the rear part of the vector ratio of the negative-sequence current of the generator terminal to the negative-sequence current of the neutral point is smaller than 0. The method has the advantages that (1) the constant value of each negative-sequence current only needs to avoid an unbalanced current during normal operation and the protection has relatively high sensitivity; and (2) a saturation problem does not exist in the optical transformer, the difference in an internal fault and an external fault is relatively obvious and the protection has relatively high sensitivity.