An analysis model was established for thermal study of infrared butt welding of the polypropylene pipes under various ambient temperature condition. One-dimensional enthalpy equation was used for the phase change heat transfer problem during the welding process. The specific heat of polypropylene, including the phase change region, was conducted according the DSC result. Infrared radiative heating was considered as the boundary condition. A correlation factor was established by comparing the analysis results from the preliminary model with the experimental results. The model with correlation factor was used for the investigation.