Abstract: Surface heat transfer coefficient is the key parameter in numerical simulation of quenching process.Surface heat transfer coefficient under quenching medium is one of the important boundary conditions for steel quenching process simulation of 42 CrMo4 steel large wind power spindle, and it is also the main parameter to evaluate the cooling capacity of quenching medium.The cooling curves of different media (water, oil) were measured by combined test.By comparing the inverse heat transfer method, numerical method and analogy method, the surface heat transfer coefficient of 42 CrMo4 steel under different quenching media was determined.Compared with the quenching simulation and the test under the same conditions, the results show that the simulated boundary conditions obtained by reverse heat transfer method have high accuracy, and the calculated values are in good agreement with the measured results.