Abstract: Using 3 mm thick 7075-T6 aluminum alloys as the research objects, this work studied the effect of a bottom-threaded pin on hook morphology and mechanical properties of friction stir lap welded joint. The results showed that the thread on the pin could significantly affect the material flow behavior during welding. The plastic material accumulated at the lap interface, squeezing the lap interface, So the hook bended downwards after welding. The width of the stir zone was obviously wider than the diameter of the pin. Because there was on thread on the tip of the pin, void easily appears at the bottom of the stir zone due to incomplete refilling of the plastic material. With increasing the welding speed, the lap shear failure load first increased and then decreased. The maximum failure load of 23.333 kN was obtained at a welding speed of 40 mm/min.