Abstract: Auto lightweight plastic products were widely applied in the automotive industry,in order to better meet the strength and appearance requirements of the plastic welding and ensure the production efficiency,PP（polypropylene） that was commonly used for the preparation of samples in auto was selected to research the effect of ultrasonic welding parameters on the strength and appearance quality of weld. The results showed that the welding depth played a decisive role in weld strength and appearance of automotive plastic parts under a certain holding time and a certain welding pressure. When the welding pressure was 0. 2 MPa,the holding time was 2. 0 s and the welding depth was 1. 8 mm,weld strength of automotive plastic parts was guaranteed and requirements of appearance quality were met. This result had certain guiding significance to the products made of PP material and ultrasonic welding process.