As protection for earth fault， the operation performance of phase-to-earth distance relay is analyzed and the mal-operation and its reason are studied when an external fault occurs on an adjacent line in the absence of a zero-sequence current on the local line. It is also shown by the analysis that there is the possibility of phase-to-earth distance relay mal-operation due to transmission line heavy load during an external interphase fault on the local line. The percentage block scheme based on zero-sequence current is proposed. RTDS simulation tests have verified the correctness of theoretical analysis and validity of the proposed method.