A 660 MW turbine generator entered a stable asynchronous running state due to loss of excitation and the generator's parameters and impedance trajectories showed regular oscillations. This paper analyzes the above process. According to the accident, a simulation model is built using Matlab to study the influence of the four factors on the process of loss of excitation, including the structure of the excitation circuit, the load before loss of excitation, the moment of inertia of the generator, and the no-load characteristic of the iron core in large-scale thermal power generating unit. The simulation results are in good agreement with those from actual case, and the correctness of the simulation model is illustrated. Simulation results have some reference significance for the loss of excitation protection of large generating units. This work is supported by Industrial Key Project of Shaanxi Province (No. 2017GY-030).