The slip device of 300 MVA motor generator (MG) was introduced. The startup process of the MG with slip mode was analyzed and modeled. According to the actual electrical waveforms, the equivalent model of the liquid resistor of the slip device was established by using the nonlinear programming model. And the MG startup process was simulated by using these models with the parameters of MG. The simulation results basically tallied with the measured waveforms. The feasibility of the models was verified. On this basis, in order to reach the rated speed of the MG, the scheme of short connection after incorporating twostage solid metal resistor into the liquid resistor was put forward. The theoretical parameters such as the resistance of the twostage solid metal resistor were given, and the feasibility of the scheme was verified by simulation. The models could be used as an important tool to study the dynamic characteristics of the MG startup process under different parameters and different control strategies.