A method and apparatus for spectrum access of secondary users in a cognitive radio system are provided. The method includes: estimating parameters of a primary channel according to the Markov model used by the primary channel; selecting a primary channel that has the largest available bandwidth according to the estimated channel parameters when multiple primary channels exist; detecting the selected primary channel; and accessing the primary channel to transmit data when the channel is idle. Therefore, an optimum primary channel can be selected from multiple primary channels by estimating channel parameters to satisfy high data transmission requirements and improve system performance.