The calculation of temperature field has a great influence upon the analysis of the thermal stresses and strains during quenching, and also upon the residual stresses and microstructure of the workpiece after quenching, too. In this paper, a 42CrMo steel cylinder was taken as an investigating example. From the TTT diagram of the 42 CrMo steel, the CCT diagram was simulated by mathematical transformation, and the volume fraction of phase constituents was calculated. The thermal physical properties were treated as the functions of temperature and the volume fraction of phase constitutents. Finally, the temperature field with phase transformation and non-linear surface heat-transfer coefficients was calculated with finite element method, and the corresponding functional of temperature was established.