With the increase in structural complexity and reliability requirements of modern industrial systems, Standby Repair Units (SRUs) are widely used in phased mission systems to recover system performance immediately. To better analyze the availability of phased series systems under the limited mission interval, cannibalization strategy or replacement strategy is considered. This paper proposes an availability analysis method based on the cannibalization strategy for the series system with equal mission intervals by considering the number of spare parts, the priority of maintenance of faulty parts, and the maintenance waiting time. Experimental results have demonstrated that this method can significantly reduce mission delay costs and effectively improve mission availability. Finally, the average availability improvement and maintenance cost reduction are discussed by considering different mission durations.