The data center is an important infrastructure for cloud service. With the rapid development of cloud computing, numerous challenges such as cross-platform requirements, network bandwidth, virtual machine migration, privacy and security emerge. Traditional topologies face some problems such as poor scalability, complicated management and inflexible communication. However, the huge cost of thousands of servers and switches is also a bottleneck for many companies to build or scale up data centers. This paper proposes a cost-efficient and flat topology, named Cell-Tree. It can achieve good performance and provide relatively high network capacity under a low cost. Accordingly, we designed a new communication mechanism and routing algorithm. The simulation shows that Cell-Tree has a relatively good performance with the cost significantly decreased.