After the practice of several HVDC transmission projects, the HVDC technique of China has been improved rapidly. In this paper, the protections of AC filters used in Tian-Guang、Gui-Guang I、II and ±800kV Yun-Guang HVDC transmission system are introduced. With the comparison of the protection used in each HVDC system, the course of development and the localization of the AC filters protection used in HVDC systems of CSG have been discussed.