According to the impact line defect of automobile outer panel, a bending-flattening-stretching method was used to simulate the formation process of impact line, and the influences of impact line on the surface hardness and morphology of samples were analyzed to obtain the relationship between hardness gradient, dent depth and bending fillet radius, tensile strain. And the influence of impact line on the appearance quality of coating was analyzed to obtain the dent depth threshold of impact line eliminating the influence, which was used as the evaluation index of impact line. Furthermore, the impact line quality problem in the material switching process of back door inner panel parts for a vehicle was analyzed and controlled by this evaluation method. The results show that the relationship between dent maximum depth and tensile strain is power exponential, and the relationship between the maximum hardness change and tensile strain as well as bending fillet radius is exponential. The impact line has a significant negative impact on the appearance quality of coating. When the dent depth of impact line does not exceed 10 μm, the influence of impact line on the appearance quality can be eliminated. By increasing the blank holder force in the forming process of back door inner plate, the impact line and wrinkling defects are eliminated, which proves the reliability and validity of the proposed impact line evaluation test index.