By analyzing and summarizing the existing test methods for isolation of cloud IaaS, a large-scale distributed testing system based on message middleware is designed and implemented for validating the isolation of a cloud IaaS.The system is a message-based distributed architecture;the control nodes and the testing nodes are separated, which helps to reduce the coupling of the system and enhance scalability.By testing the isolation of cloud IaaS in operation, the feasibility of the testing system is verified on the OpenStack platform.The testing system is suitable for testing other security abilities of cloud services.