The temperature drop process of waxy crude oil in an oil tank is an unsteady natural convection process involving multi-substance coupling, multi-heat transfer coupling, phase transformation, solid-liquid interaction, rheological change, fluid-solid coupling and turbulence, and the heat transfer process is highly complicated. According to the influence of the phase change process of waxy crude oil on the heat transfer process, based on the new three-phase partition method, the phase change heat transfer model of waxy crude oil in an oil tank was established, and the finite volume method was used to carry out the equation discretization and solution. Through numerical simulation, the evolution laws of the temperature field, flow field, and growth of the condensate layer during the cooling process of oil in the tank were analyzed. The results had theoretical guidance and significance for the scientific design of the oil tank's thermal insulation structure.