The existing starting mode of diesel locomotive has the characteristics of high equipment space occupancy,low transmission efficiency and high failure rate.Under the technical background of upgrading of diesel locomotive,this paper proposes to use synchronous generator to start diesel engine,which is coaxial with diesel engine and can run in the motor condition,and studies the speed characteristics of the motor condition.Based on the analysis of the working principle and the boundary conditions of the starting condition of the variable frequency diesel engine,the state equations of the starting condition of the motor are derived.In order to shorten the solving time and improve the solving precision, Runge-Kuta method is introduced to solve the above equations.The simulation analysis software is written to analyze the starting condition of the alternator.The results show that the mechanical torque of synchronous motor is greater than the maximum resistance torque of diesel engine 12000 N·m,and can reach 210 r/min in 12 s,which can meet the requirements of starting conditions.The analysis process can also be used to check the control strategy and start-up capability of the diesel generator set.