We propose and analyze an unslotted CSMA with double back-off in multi-channel cognitive radio network. We model the system by 3-demensional continuous time Markov process, calculate the steady state probability of the system, and obtain several performance measures such as throughput and forced termination probability of SUs. By comparing the performances of CSMA scheme with single back-off and double back-off, the numerical results show that CSMA scheme with double back-off overcome the defect of low performance in CSMA scheme with single back-off.