The heavy 125MN Plunger is weight 140 ton, its heat treatment process include two part, one is heat and another is quenching in order to attain certain hardness on its surface and toughness on its body. Differential temperature heat process were made in well resistance furnace, the heat time include warm-up and holding period and high-speed increasing period. The total time is about 30-40 hours.The differential heat treatment process is an essential step in the production of heavy plunger, and it determines the hardness distribution on the sample surface. In this paper, a program to calculate real-time temperature during the heat treatment process was developed based on a great deal of experiments. Using this subroutine program the temperature field of hollow heavy plunger during differential heat treatment process was calculated. The result shows that the simulated temperature was agreed with the real temperature in the warm-up and holding period of the heat treatment process. In the calculation temperature field ,based on the convectional heat transfer coefficient and radiate heat transfer coefficient, the corrected heat transfer coefficient were induced to compose comprehensive heat transfer coefficient. The temperature predicted in warm-up and holding period were correct, the error was below 1%. At the high-speed increasing period, the time when the external temperature reach enacting was predict accurately. So the computer simulation can give real time prediction to decide parameters of the heat treatment process. The result also show that the program developed is sample and applied , it fit for predicting temperature at workshop real time and it is available to deal with complex process .