Abstract: The paper introduced testing method of mean time between failures, did site test of mean time between failures in four coal mines to the same coal mine monitoring system, and got mean time between failures of main equipments of the monitoring system. It concluded: the values of mean time between failures of intrinsic safety power, gas sensor, CO sensor and wind speed sensor are relatively small, and these equipments should be the focus of future design and improvement; the monitoring system can achieve the index suggested in AQ 6201—2006 that mean time between failures should be not less than 800 hours; statistical results show that line fault of underground coal mine occurs frequently.