To solve the problem of transient thermal simulation consuming a large amount of computer resources, model data of IGBT air-cooled radiator based on transient heat conduction equation and model order redution method were extracted by computational fulid dynamic method and finite element method. The condensation degree of freedom of reduced order model was set up by using Krylov subspace projection method and then calculated. The simulation results indicated that the reduced order model could accurately reflect the characteristics of physical fields, the model order reduction method could not only meet the accuracy requirement, also can greatly reduce the computational complexity and improve the calculation speed.