On the magnetic field characteristics of generator with single phase short circuit, take a nuclear power half speed turbo-generator as an example, the simulation model of generator single phase short circuit are established by field circuit coupling method. Through the simulation of normal and single phase short circuit of single generator under no load condition, the distribution of the internal magnetic field and the variation characteristics of the air gap flux density before and after the short circuit are obtained. It is pointed out that the magnetic field of the generator is still symmetrical after fault. The air gap flux density is distorted after the fault，Through FFT，the results show that the fundamental component of air gap flux density decreases obviously, while the odd harmonic component increases.