The influence of clamp pressure on the gap, the depth of fusion and the lap-shear strength of weldments is studied based on PA66 thermoplastic materials reinforced with glass fiber and carbon fiber. The relation among the gap, the depth of fusion and the lap-shear strength is analyzed, from which the great importance of clamp pressure in the welding test can be seen. The effect of welding velocity and power on the depth of fusion and the lap-shear strength is then studied deeply. The connection between the depth of fusion and the lap-shear strength of weldments is obtained from a large number of experimental data and fitting curves. Finally, the influence of linear energy on the depth of fusion and the lap-shear strength of weldments is discussed. The results show that the process parameters affect the weldments gap and the depth of fusion, which results in the changes of the lap-shear strength. The study of the changes of the weldments gap and the depth of fusion can make it clear that how does the process parameters affect the lap-shear strength.