To address the problem of zero sequence direction relay's non-operating under the zero sequence voltage's threshold， a new method of phase comparison is presented by using positive sequence voltage instead of zero sequence voltage. Based on the systematic exposition of phase deviation of positive sequence voltage between pre and post fault， the effects of fault resistance to positive sequence， negative sequence and zero sequence component， the maximum phase deviation of positive sequence voltage and negative sequence voltage caused by fault resistance， and the effects of sending and receiving ends power-angle of power grid on the phase deviation are systematically described when asymmetrical earth fault occurs. The research result shows that when using the new method， mal-operation will not happen under external faults， and operation will be correct under internal faults except some special cases.