According to thermodynamics and kinetics of phase transformation, a mathematical model of phase transformation was proposed, and the microstructure evolution of an 82B rod at different cooling rates was simulated by using the FEM software Marc/Mentat, based on the measurement of time-temperature transformation (TTT) curves of the 82B rod. The simulated results were in good agreement with the actual measurements. From the results of computer simulation it was found that the cooling rate of the 82B rod, after laying, should be controlled within 5-8℃/s. In the microstructure of rod there were over 95% of pearlite volume fraction and a small quantity of dispersive martensite (less than 5%).