AC excitation motors have the advantages of constant frequency in variable speed and power decoupling in steady-state, which have significant advantages in applications such as pumped storage and flywheel energy storage. However, the above scenarios require motors have ability to brake in emergency with large inertia load, and the traditional mechanical braking strategies don’t satisfy the requirements. A flexible braking strategy and control parameter optimization method are proposed for motors with large inertia load. Firstly, based on the structural characteristics of rotor, flexible braking strategy is proposed that the rotor connect to DC excitation power and the stator connect to multi-stage resistors, then derive the equivalent circuit in braking procedure. Secondly, establish braking parameter optimization model with multi-stage braking resistance, rotor excitation current, and speed of switching resistance as variables, rated motor parameters and braking resistance power as constraints, and the shortest braking time as the objective, then use genetic algorithm to solve the model. Finally, analyze the multi-stage resistance braking results and influencing factors through Matlab/Simulink simulation, and establish 7kW AC excitation motor platform to verify the simulation results. The results show that adopting multi-stage resistance flexible braking strategy can effectively reduce braking time, and optimizing braking parameters can achieve the shortest braking time and satisfy system power constraints, while balancing braking effectiveness and the economy of braking device.