The melted plastic has high viscosity and low thermal conductivity. In the field of thermal disposal, the melting behavior significantly impacts reactor heat transfer and operational performance. In this study, it was experimentally found that the pyrolysis of plastics was characterized by three stages of "melting-boiling-pyrolysis", in which the phase change enthalpy absorption caused significant changes in the reactor's temperature distribution. A three-dimensional melting model for plastic particles was developed based on the volume of fluid (VOF) model coupled with the enthalpy-porosity method. This model simulated the melting behavior of plastic particles under hot gas flow heating conditions, focusing on analyzing the effects of particle size and shape on temperature distribution, melting rate, and interfacial heat transfer during melting. It was found that there was a distinct internal temperature gradient within particles during melting, with the melting rate initially increasing and then decreasing over time. Compared to spherical particles with a diameter of 10 mm, increasing the particle diameter by 50% extended the melting time by approximately 20 seconds. The heat flow at the gas-liquid interface during melting initially increased and then decreased over time. The total heat required for complete melting was proportional to mass. The total heat for melting 20 mm spherical particles was approximately eight times that of 10 mm particles. The melting rate of different shapes of particles was affected by the windward area and specific surface area, etc. At the same mass, cylindrical and rectangular particles had faster melting rate than spherical particles, and the melting time was reduced by 22.7% and 18.2%, respectively, compared with that of spherical particles. The results provide a theoretical basis for the optimization of pyrolysis reactor design.