The grounding technology of the secondary system of substation relay protection is an important measure to ensure the reliability of relay protection devices and the stable operation of the system in the power system. The grounding of the secondary system can effectively resist lightning interference， primary system interference， operation interference， and partial discharge interference， improving the safety and stability of the power system. This article provides an in depth analysis of the grounding technology for the secondary system of substation relay protection， and explores the specific applications of secondary cable shielding layer grounding， AC secondary circuit equipotential grounding， secondary equipment equipotential grounding， and outdoor terminal box equipotential grounding， aiming to improve the anti interference ability and operational reliability of the power system.