This paper presents RacsCBPS, an admission control scheme for large-scale and scalable content-based publish/subscribe systems. First the key requirements to implement admission control in content-based publish/subscribe systems are identified and how it differs from admission control schemes in the Internet and other research areas is analyzed. A cover-relation based algorithm to compute subscription resource requirements and an admission control algorithm based on subscription routing are presented. The scheme ensures time, space and control decoupling without sacrificing scalability of publish/subscribe systems. Publish/Subscribe systems can seek different balances between system resource utilization and QoS guarantee by choosing different admission control criteria. Experimental results show the effectiveness of the method.