This paper proposes a two-dimensional finite element mathematical model taking the mf-15 simulation generator in the dynamic model laboratory as the research object. The electromagnetic performance of the motor under no-load and rated load was analyzed to verify the correctness of the model. By using the method of field-circuit coupling combined with simulation analysis, the limit value of the generator`s phase-in depth is determined, and the variation law of the electric and magnetic field distribution with the depth of the generator`s established Leading phase operation is analyzed, as well as the influence of the phase-in operation on the system`s operation stability. The results obtained provide a reference for the phase-in operation analysis of large generators.