Based on application of differential projection operator for solution of distributed parameter telegraph equations of the line， a novel principle for the single-ended fault locating is proposed. According to the analytical solution， a new fault locating function is constructed， which has only a minimum value at the fault point. Whilst， the fault locating functions calculated with real voltage value and with fictive voltage have similar distribution. Hence， the fault can be determined by utilizing the fictive single-ended datum， combined with the fault locating function. Then three-phase fault locating algorithm is described in detail. The property and accuracy of new fault locating principle is verified with EMTP simulation tests.