The invention provides a fairness-based power and channel joint allocation method for a cognitive radio system, belongs to the technical field of wireless communication, and in particular relates to power and channel joint allocation used in the cognitive radio system. The invention relates to the power and channel joint allocation method which can meet the requirements of unauthorized users on quality of service (QoS), ensure the fairness and protect the authorized users at the same time. According to different QoS requirements of the unauthorized users, a mathematical definition on the fairness is given. Simultaneously, in order to reduce arithmetic cost and shorten the allocation period, the method divides the whole allocation process into two stages; the first stage is called as the crude allocation, at the stage, the protection of the authorized users is taken as premises, and the maximum resource which can be utilized by each non-authorized user is obtained according to the QoS requirements of the non-authorized users and the fairness; and the second stage is called as the fine allocation, at the stage, a plurality of non-authorized users independently complete the tasks, which can be serially and simultaneously completed by the authorized users, so that the complexity is reduced, the quick power and frequency spectrum allocation can be conveniently realized so as to adapt to the quick time varying characteristics of available frequency spectrum resources in the cognitive radio system, and all authorized users can independently determine the final utilizable resources.