Wax precipitation has proven to be a serious problem when extracting the crude oil from current oilfields. Thermal washing is one of the common methods to remove wax deposition in oil wells. Mastering the melting process and the melting time of the wax layer in thermal washing is essential to optimize operating parameters and further improve the efficiency of thermal washing. In this paper, the scouring model and the melting model are carried out to simulate wax melting behaviour during the thermal washing process in a vertical oil well. Furthermore, with the melting model, the effects of five factors on the melting time have been studied. The results reveal that the melting time is mainly influenced by the thickness of the wax layer and the water flow rate, while the initial temperature of the wax layer, water temperature and inner wall temperature of tubing these three temperatures have a relatively slight influence on the melting time. Finally, with the numerical simulation results, a correlation is developed to predict wax melting time and verified with thermal washing time of the Cha well. It indicates that the melting model is applicable for most of the waxing section, while the scouring model is only applicable when the water initially reaches the bottom of the waxing section, so the melting model is more practical for thermal washing in the oil well.