Absorber-free laser transmission welding of PVC (polyvinyl chloride) and PC (polycarbonate) was conducted by using a 1 710 nm semiconductor laser. The welding specimens with ideal welding effect was obtained through designing multiple comparative tests by taking laser power, welding speed, and defocus distance as three factors that affecting the welding results. The results show that the welding specimens has the flat surface, uniform weld seam without bubbles, and the largest breaking force when the laser power of 20 W, welding speed of 10 mm/s, and the defocus distance of -4 mm.