The invention discloses a distance protection method for a power transmission line. A to-be-solved problem at present is how to further improve the distance protection action speed. The method comprises the steps of carrying out same low-pass filtering treatment on three-phase measurement voltage and current and memory voltage at a protection installation position; detecting whether measurement voltage at the protection installation position is lower than a threshold value or not after a fault, if the measurement voltage is higher than the threshold value, selecting an appropriate data windowlength, building a series of differential equation sets on the basis of a line model, obtaining a fault distance through solving differential equations and comparing the fault distance with a set value; and if the measurement voltage at the protection installation position is lower than the threshold value after the fault, replacing equivalent power voltage at the back side of the protection installation position with the memory voltage on the basis of the line model, selecting the appropriate data window length, building a series of differential equations and obtaining the fault distance through solving the differential equations. According to the distance protection method, fast distance protection action of the power transmission line can be achieved, the problem in direction distinguishing of three-phase faults at an outlet can be effectively solved and the safety and stability margins of a system are effectively improved.