Abstract: Relevant parameters of a pressurized water reactor are obtained to establish the 3D geometric model and two-dimensional heat conduction differential equation. To calculate the temperature distribution and melting process, the natural convection heat transfer model and radiation heat transfer with neighbor 8-rod-cavity model are applied. The study shows that with the accident processes, the water level in the core decreases, but the temperature increases, and the point with the highest temperature in the core goes down gradually. After 560 seconds, the control rods begin to melt, and after 1200 seconds, the stainless steel rods begin to melt; the fuel rods begin to melt after 2700 seconds. About 7000 seconds later, the proportion of core melt is more than 50%. When most of the core nodes melted, the shroud didn’t melt. The melt migrated directly down the lower plenum. Both the steam vapor and radiation can influence the melting time of the fuel rods, and the steam convection heat transfer is dominant. The effect of steam vapor cannot be neglected. Radiation heat transfer has the effect of flattening the temperature of the core.