In this paper,we introduce a new kind of homomorphic signature,which is suitable for identity authentication in cloud computing.A user firstly computes the full signature on its all identity attributes,and sends the full signature to the intermediate channel server (the intermediate channel server should check and verify it).During the valid period of his full signature,if the user wants to require a cloud service on a special identity (only involves part of identity attributes),he only needs to securely send a {0,1}n vector to the intermediate channel server.The intermediate channel server who doesnt know the secret key can compute the partial signature on the special identity,and then sends it to the cloud server.We give a formal secure definition of this homomorphic signature,and construct a scheme from GHR signature.We prove that our scheme is secure under GHR signature.