Abstract: In view of problems of low reliability and accuracy of fault diagnosis of asynchronous motor in bolter-miner, symmetrical component method was purposed to apply in protection system of bolter-miner to realize asymmetry over-current fault diagnosis. Three-phase voltage is decomposed into positive sequence component, negative sequence component and zero sequence component, then active phase shifting circuit and phase sequence judgment circuit are used to judge phase sequence of main circuit, and the negative sequence voltage is obtained by negative sequence component acquisition circuit and is used to judge fault and its type. Industrial test results show that application of the symmetrical component method improves sensitivity, reliability, accuracy and intelligence level of the protection system of bolter-miner, reduces system maintenance cost, and improves service life of asynchronous motor and excavation speed on working face.