A method for authenticating a terminal subscriber identity module in an IM domain includes: sending an AV request to an HSS by the S-CSCF; sending a request message for acquiring the AV to a home device of the UE upon receiving the AV request by the HSS; allocating an SQN for the UE upon receiving the request message, generating an AV based on the SQN, and sending the AV to the HSS by the HLR; sending the AV to the S-CSCF by the HSS; and authenticating the terminal subscriber identity module of the UE according to the AV by the S-CSCF. A system for authenticating a terminal subscriber identity module in an IM domain includes: UE, an S-CSCF, a home device of the UE and an HSS. The invention enables a USIM subscriber to use IM services without changing the USIM card.