With the development of cloud computing, cloud platforms of different types and purposes are emerging. Hybrid clouds are needed to manage cross-domain computing and storage resources in a unified manner in order to satisfy management requirements such as legacy system integration and dynamic resource scaling. However, there are differences in management interfaces and mechanism between different cloud platforms, which cause great difficulties and high complexity to construction of hybrid clouds. In this paper, a runtime model based approach to managing hybrid clouds is presented. First, the manageability of cloud platforms is abstracted as runtime models based on their management interfaces. Second, a unified model of cloud software architecture is provided according to the domain knowledge of current cloud platforms. Third, the synchronization between the unified model and cloud runtime models is ensured through model transformation. Thus, all the management logic can be carried out by executing programs on the unified model, which decreases the difficulty and complexity of hybrid cloud construction. The experiment on a real-world hybrid cloud, which consists of CloudStack and Amazon EC2, demonstrates the feasibility, effectiveness and benefits of the new approach in managing hybrid clouds.