<p>A method for detecting the QoS abnormal is provided. The method includes: acquiring the QoS index transmitted from the parent peer data flow, detecting the parent peer QoS abnormal according to the QoS index and the corresponding QoS index domain value. A method for detecting the factor resulting in the QoS abnormal is also provided. The method includes: detecting whether the relating peer QoS is abnormal or not, statistic the QoS abnormal proportion of the relating peer, and determining the factor resulting in the QoS abnormal according to the abnormal proportion of the relating peer. Accordingly, a system for detecting the QoS abnormal anda system for detecting the factor resulting in the QoS abnormal are also provided.</p>
