Digital signature provides origination, integrity and non-repudiation. Multisignature is a special type of digital signature, where multiple users cooperate to sign one message. On the other hand, RSA is a popular cryptosystem, and using identities to be public keys is a convenient approach for identification. So, Harm and Ren proposed an identity-based RSA multisignature scheme which combines Shamir's identitybased signature scheme in 2008. Later, Chang et al. indicated that Harm and Ren's scheme suffers from some drawbacks in 2009. In this manuscript, we will propose a secure identity-based RSA multisignature scheme to improve Harm and Ren's scheme. Moreover, identity-based public keys will not place extra burden on users because users do not need to verify certificates in advance. The proposed multisignature scheme can be applied for mobile communications.