Abstract: In view of problem that fault type and phase of asymmetric voltage sag could not be quickly identified by traditional voltage sag detection algorithm, a voltage detection method based on dq transformation and symmetric component method was proposed. Firstly, the voltage is decomposed by symmetrical component method, and the asymmetrical voltage is decomposed into symmetrical positive, negative and zero sequence components. Then the decomposed components are transformed into synchronous rotating coordinate system dq to obtain the amplitude and phase information of positive, negative sequence and zero sequence components of voltage components under various faults. Finally, the results of the transformation are reorganized. Taking voltage sag caused by short circuit fault with high frequency as an example, the short circuit fault voltage is transformed and reorganized. According to the recombination results, not only the fault type can be identified, but also the specific fault phase can be located. The simulation comparison with the traditional dq voltage transformation method verifies accuracy and superiority of the method.