Polymethyl methacrylate (PMMA) transparent plaque and polystyrene (PS) semitransparent plaque were jointed together by laser welding，and the tensile shear strength of the resultant samples were examined by two widely accepted testing methods，simple-lap method without additional joint and simple-lap method with additional joint.The comparative testing results show that simple-lap joint method without additional joint can not keep the stress at vertical definition to the plaque surface during testing，and the simple-lap method with additional joint also fails in shielding the specimens from tensile stress during testing，due to the existence of additional bending moments.It implies that the additional lap joint at the fixture block is not successful in providing pure shearing stress condition for the plaques.And the main factors of the deflection distortion angle () caused by additional bending moments are also discussed.