College of Computer Science and Technology, Harbin Engineer University, Harbin 150001, ChinaAbstract: According to randomness of service failure for high dynamicity of cognitive networks, a service migration method is proposed to ensure QoS of cognitive networks. Firstly, with the principle of optimization-after-migration, the directed acyclic graph (DAG) of correlated service is regenerated according to the proposed DAG dynamic reconstruction algorithm to transform the correlated service to layered DAG service. Secondly, the critical service migration route is computed and the analysis of migration service deadlock avoidance is provided. By migrating critical service to current idle resources, service execution time can be reduced markedly. Finally, simulation experiments are conducted to test the service speedup performance of both service migration method and waiting-recovery method with three kinds of faults injected. The experiment results show that service migration method can achieve better QoS assurance quality under the flexible network load and unknown fault injection.