To solve the problem of calculating the transient temperature field of motor under steady load, taking the small enclosed structure of the asynchronous motor as the research object, the calculation of heat source in the motor electromagnetic field model is established by finite element methods, the corresponding temperature field model is established according to the characteristics of enclosed structure, the combined calculation of electromagnetic field and temperature field is realized by means of multifield coupling, attached to the model parameters related to the electromagnetic field and temperature field and the coupling parameters of the joint calculation, through doing combined calculation of electromagnetic field coupling temperature field properly, realizing the calculation of each transient temperature field under steady load. Through the actual system test, comparing between the global temperature field experiment of the motor under steady load and simulation results, multifield coupling calculation method can realize the calculation of the motor whole transient temperature field under steady load reliably.