The imprecision of network state has to be considered in the QoS routing because of non-negligible propagation delay of state messages, periodic updates due to overhead concern, and hierarchical state aggregation. A probability-based QoS routing algorithm is presented in the paper. The premise-controlled sub-optimal algorithm can find delay-bandwidth constrained least cost route when only imprecise information available. Experimental results demonstrate that the algorithm can shield the imprecision of network state and tolerate the insensitivity of the triggering methods with good routing performance.