In the first UHV AC testing demonstration project of China, 1000kV Jindongnan Substation adopts 1100kV gas-insulated metal-enclosed switchgear (hereinafter abbreviated as GIS), which is the first UHV AC GIS operating commercially in the world. Because of high voltage and large capacity, the design of the earthing system for this equipment possesses new characteristics, and concerning the actual construction conditions we proposed the concept of setting auxiliary ground grid. By analyzing and calculating the configuration of GIS, number of earthing point, power-frequency short-circuit current and lightning impulse current probably through, quantity of ultra-high frequency current, finally, we confirmed the size of the auxiliary grid. This article shows the earthing current value through each position of GIS and which is verified by current-resolution.