With increasing critical business organizations focusing on the quality of service in cloud applications, high availability (HA) has become critical for the product level cluster of cloud applications. Existing availability evaluation methods or testing projects for cloud platforms, however, cannot sufficiently verify cluster tolerance to various reasonable environmental faults. This paper proposes a test case design method targeting environmental fault tolerance for HA clusters. Test cases are designed considering fault modes and components of a cluster application in diverse scenarios. As a result, the fault tolerance of different components to various reasonable faults can be verified via the proposed method. A case study is conducted on Openstack, a widely used open source software platform for cloud environments, to show the feasibility of using this method.