In this paper, a novel time-domain algorithm for long transmission line distance protection is proposed. When a certain fault occurs, the current and voltage at the end of the protection zone can be calculated by using the measured current and voltage at the relay. By applying the new algorithm, which has three coefficients needed to be resolved by time-domain differential equation and fits long distance transmission line, to the current and voltage data at the end of the protection zone, we will obtain the fault distance. When the computed distance is less than the setting value, we consider the fault as an internal fault. Theoretical analysis and simulation prove that this method is not affected by the distributed capacitance or aperiodic component. Furthermore, the algorithm works very reliably without overreaching and its speed is really fast. As to a 300km long transmission line with a 300Ω transition resistance, this algorithm is verified to be valid and accurate.