Based on the research on Tian-Guang, Gao-Zhao, Xing-An and Yun-Guang I HVDC transmission projects, metallic return longitudinal differential protection is first used on Pu-Qiao UHVDC Transmission for the metallic line fault in metallic return operation. In this paper, the principle of metallic return longitudinal differential protection is described in detail and the cooperation of backup line fault protection is introduced first; then, the experiment is did on platform composed of RTDS and the control and protection system of Pu-Qiao UHVDC Transmission. The experiment results show that the new protection is effectual and legitimate cooperated with backup protection. So the used of the new protection can reduce the system stop by metallic line fault effective and improve the reliability of system running. Now, metallic return longitudinal differential protection is executed on Pu-Qiao UHVDC Transmission protecting system, the operation is normal.