In the car painting process,the dirty point of the painting is a key KPI for paint shop,because the finish line have to repair every dirty point of the area where could be seen by the customer.Too much dirty point will cause the long quality stop in the finish line and more car need spot repair.In this article,we try to find the root cause from the aspect of man,machine,material,method and environment,we hope it could help the people in the other painting workshop.