Reducing the square of lap joint by using lap welding of four-layer sheets achieves the lightweight of auto-body in foreignproduction cars. To get the technology,experimental study on spot welding joint of four-layer sheets of body-in-white in a domestic car design was done. The results show that the spot welding defects of four-layer sheets can be prevented and reduced effectively by optimizing welding process and improving assembly accuracy. The method can be applied appropriately when it meets the quality requirements of the company. For four-layer sheets with 0.7 mm thickness per sheet,under the spot welding parameters of preload electrode force of 2.0 kN, welding electrode force of 2.6 kN,welding time of 230 ms and welding current of 10 kA,the welded joints have good appearance and optimum mechanical properties. The results can provide technical guidance for later mass production.