Considering the characteristics of IEC 61850 protocol， features of digital substation communication and special requirements for earth fault line selection， a scheme for earth fault line selection based on digital substation technology is proposed. Three key elements that affect the correctness of line selection most is analyzed， including precision of electronic instrument transformers when merging zero sequence current， control on synchronized sampling of several merging units， and communication mode. Considering the feature of electronic instrument transformers， transient-based algorithm of line selection is the first choice in the digital substation environment. Meanwhile， generic object oriented substation event（GOOSE） communication technology is adopted to realize tripping function.