Linear defects is one of the common surface defects of cold-rolled sheet. It influences cold-rolled sheet quality and function seriously, while "black line" defect is one of the two linear defects. The simulating rolling experiment was done through the simples which are collected on site. The "black line" defect produced in the simulating rolling experiment is photographed. By observing surface quality, and analyzing composition of the defect and inclusions around with the EDS, the measures to reduce "black line" defect were put forward and the formation mechanism was discussed on the basis of a comparison with the defect under normal craft.