0.1-mm-thick PET films and 0.1-mm-thick titanium are joint using laser transmission joining technology. A 3D thermal model based on the finite element theory to simulate the temperature field on laser transmission joining of PET and titanium is established. With the ANSYS parametric design language (APDL) a moving super Gaussian distributed heat source is applied,and the distribution of the temperature field and the time to achieve quasi-steady-state temperature field are gained. Effect of process parameters,including laser power and scanning welding,on the joinging result is investigated,and the calculated joint width is obtained. Experimental data for joint width is collected. Very good agreement is achieved between the calculated values and the measured values. It shows that the model is reliable.