For the threephase asynchronous motorgenerator of a “steamelectric doubledrive” system of shanghai Motor Factory Co., Ltd., which was 6 800 kW, 750 r/min, the transient performance was analyzed based on Ansoft software using fieldcircuit coupling method. Analyses and calculations were carried out for three different conditions: noload start, fan load start, and big load start. The process of the motor being switched from the state of motor to the state of generator was simulated. Instantaneous starting current, electromagnetic torque, the air gap flux density and other electrical parameters of the threephase asynchronous motorgenerator in different states were obtained. The transient performance of the asynchronous motor and the corresponding electromagnetic field distribution were analyzed.