The same transparent polyethylene terephthalate (PET) plastic was welded by laser transmission welding using the 1 710 nm diode laser. Because the transparent PET plastic has a high absorption rate for 1.7 μm laser, there is no need to use laser absorber in the welding process. The influence of energy input on welding strength was discussed by adjusting laser power and welding speed. When the laser power is 10 W and the welding speed is 7.5 mm/s, the aesthetic and strong welding parts are obtained with the energy input is 1.33 J/mm. Keeping this energy input constant, a comparative experiment was designed by changing laser power in the range of 5-30 W. Results show that the change of laser power (5-30 W) and welding speed (3.75-22.50 mm/s) has little effect on the weld strength of the welding specimen.