In order to study the variation characteristics of electromagnetic torque of generator under single phase short circuit, taking a nuclear power generator as an example, The simulation model is established by field-circuit coupled method. Through the single phase short circuit simulation of generator with rated load, the variation characteristics of air gap magnetic field and electromagnetic torque before and after the fault are obtained, the harmonic components of electromagnetic torque before and after the fault is analyzed. The results show that the DC component of the electromagnetic torque decreases after the fault, and even harmonic components are generated.