A three-dimensional (3-D) thermal model based on the finite element theory is implemented to simulate the temperature field on laser transmission welding of polyvinylchloride (PVC) by means of software ANSYS. With the ANSYS parametric design language APDL, a moving Gaussian distributed heat source is applied, and the distribution of the temperature field is gained. Effect of process parameters, including laser power, scanning welding and spot diameter, on the welding result is investigated, and the calculated melting width is obtained. The results show that with increasing of scanning speed, the maximum temperature and the melt width decrease; with increasing of laser average power, the maximum temperature and the melt width increase; with increasing of spot diameter, the maximum temperature decreases and the melt width increases. Very good agreement is achieved between the calculated value and the measured value.