Under the assumption of equal electromotive force（EMF） amplitude ratio of both ends in power systems， based on the pre-fault phase relationship between EMF and load current the contra-lateral source impedance is legitimately estimated. With measured impedance of traditional distance protection and real-time phasors on protection location， adaptive math models for calculating both resistance and reactance of line fault are established. Both models are functions of positive-sequence current branch coefficient phase angle. According to increase or decrease relationship between reactance， resistance components of measured impedance and positive-sequence current branch coefficient phase angle， two cases solutions are separately proposed in order to prevent overreaching and under-reach problems of distance relay. Simulation results show that the adaptive distance protection has satisfactory performance， enhanced reliability and high sensitivity.