The rapid development of cloud computing brings a new concept of green cloud computing that needs to make the cloud consuming less resources at the time of services providing. The user authentication in cloud computing is very important, which can ensure the security of the cloud access and avoid the leakage of the data in the cloud. Therefore, an efficient cloud user authentication protocol is required to satisfy development requirements of the green cloud computing. In this paper, we resort to the tree-based signature to design a novel authentication protocol for green cloud computing, which can improve the efficiency and guarantee the security. Note that the proposed protocol can provide many security properties such as privacy preserving, forgery attack resistance and replay attack resistance due to the superior performance of the tree-based signature. The performance analysis shows that the proposed protocol is more efficient than previous protocols.