The initial temperature of the preform has an important influence on the stretch and blowing step of the process to produce PET bottles. A complete 3D modelling of the heat part of the stretch blow molding machine including meshing is a long and complex task. Solving Navier Stokes equation coupled with the thermal problem takes more than one week using ANSYS/Fluent software. The numerical simulation of infrared (IR) heating taking into account the ventilation effect is very time-consuming. This work proposes a simplified approach to achieve quickly the numerical simulation in order to have an estimation of the temperature distribution in the preform. In this approach, the IR heating flux coming from IR lamps and the ventilation model are calculated in a semi analytical way and are applied as the boundary conditions of the simulation in COMSOL where only the preform is meshed. This approach is validated by comparing our numerical results with the experimental temperature distribution of PET preform.