Thermal simulation technology was employed to investigate phase transformation in heat-affected zones (HAZ) of high Cr ferritic heat-resistant steel. The simulated continuous cooling transformation diagram was established based on the experimental results obtained from different cooling rates in the range of 0.02-60 degrees C/s. A theoretical model considering the site saturation nucleation at grain boundaries has been applied to calculate the austenite fraction as a function of cooling rate. It is found that both the austenite fraction and grain size decrease with the increase of cooling rates. The calculated results are mostly consistent with the experimental data.