A method and device for authenticating a MS has an R-UIM by using CAVE Algorithm are provided. The hardware structure of the device includes a cdma2000/HRPD dual-mode chip, a User Identity Module supporting the CAVE algorithm. The dual-mode terminal forms the NAI value with the domain name stored in a memory of the dual-mode terminal in advance by the IMSI. The dual-mode terminal extracts a RAND that is necessary for the calculation of an authentication parameter1 from the Random values included in a Chap Challenge message, instructs the R-UIM card to use the CAVE algorithm to calculate the authentication parameter1 with the RAND and an existing SSD_A in the R-UIM card, and bears the authentication parameter1 by the Result domain of a Chap Response message. With the present invention, the wastes caused by the replacement of R-UIM cards can be avoided.