The extension of digital substation is different with the conventional, where the generic object oriented substation event (GOOSE) information of the entire intelligent electronic device (IED) is exchanged through switch, and the debugging device may access to same switch with other operating device. So it must eliminate the impact of the new equipment to operating GOOSE network and ensure the integrity of the secondary experiment for new devices, in the expansion process. Taking the interval extension of Xin’an I line in Zhiyan 500 kV substation for an example, this paper discussed the safety measures of new equipment and the implementation of debugging program in substation extension process.