As more and more large-scale scientific workflows are delivered to clouds,the business model of workflow-as-a-service is emerging.But there are many kinds of threats in the cloud environment,which can interrupt the task execution and extend the workflow completion time.As an important QoS parameter,the workflow completion time is determined by the critical task path.Therefore,critical path redundancy method is proposed to create a redundant path having the interact parallel relationship with the critical path,which can provide the protection for the tasks in the critical path and reduce the probability of the critical path interruption.Computing instance allocation is an essential part of the cloud workflow execution,since only the tasks assigned the instance can begin execution.In order to further reduce the workflow completion time,computing instance allocation algorithm based on HEFT (heterogeneous earliest finish time) is proposed.The algorithm considers diverse task dependency relationships and takes full advantages of the critical path redundancy method,which can improve the efficiency of workflow execution.Experimental results demonstrate that the proposed method can effectively reduce the cloud workflow completion time under the task interruption.