An improved tensile shear strength testing method for plastic of laser transmission welding(LTW）is proposed. The single-lap weld with two sheets and one seam is changed into double-lap weld with four symmetrical sheets and four seams. Transparent Polypropylene (PP) and the opaque Polypropylene (PP) are jointed together by two methods, single-lap joint and double-lap joint. The tensile shear strength of the resultant samples is examined respectively. The comparative testing results show that, there is no deflection distortion in the weld seam of double-lap specimen without additional bending moments, while there is certain deflection distortion angle in single-lap joint, due to the additional bending moments in the tensile shearing test process. The test value of shearing strength of double-lap joint specimen is generally higher than single-lap joint in the same energy density range, because the single-lap joint seam fractures in the un-synchronous way, while the double-lap joint seam fractures in the synchronous way.