The invention provides a voltage drop-based distribution network fault detection method. The method mainly comprises the steps of sampling a voltage signal after a fault by adaptive sine digital filtering; by adopting the least square method to calculate and obtain an instantaneous amplitude value of each phase of voltage; and judging the fault type according to the calculation result. By taking voltage change rate as the distribution network fault detection criterion, influence of distributed power supply to current protection can be lowered effectively, the distribution network fault can bejudged accurately, and an outstanding practical value for improving correctness of the protection actions is achieved.