In system design process, standby redundancy is a widely used technique to improve system reliability and availability. Typical standby techniques involve cold standby, hot standby, and warm standby. In this article, we investigate the repairable K-out-of-N system with mixed standby strategy containing both warm and cold standby. In the proposed system, each component can be in failure, cold, warm, and active states and the components are assumed to be repairable. The systems are modeled by continuous time Markov chain and the system long-run availability is derived. Furthermore, the optimal configuration of standby components in the system is studied considering both system availability and system running cost. Illustrative examples are presented to show the applications of the proposed model.