A method and a device for reporting an identity are provided according to embodiments of the present disclosure. The method includes: when determining that the number of bits of a temporary mobile subscriber identity (S-TMSI) is greater than M, selecting M bits from the bits of the S-TMSI, M being a predetermined positive integer; and transmitting a first message 3 (MSG 3) to a network device, where the first MSG 3 carries the M bits.