A wireless communication system using channel access based on a contention approach and method thereof is disclosed, which allocates plural sets of time slots in a predetermined contention interval, wherein each set of time slots further includes plural adjacent overlapped time slots. A channel requester transmits a contention message to a channel allocator at a time slot based on an interleaved method after the channel requester completes count down of a predetermined time, so that the positions of the headers of the contention messages in the adjacent time slots of each set of time slot are different. Thus, if the contention message transmitted by the channel requester collides with the contention message of the adjacent time slot, the channel allocator can parse the headers of the collided contention messages and respectively allocate plural special channel times for communicating with the channel requesters.