A method for determining a back-up frequency in a cognitive radio system comprising a first cognitive system, a second cognitive system and an incumbent system, wherein the method is characterised by: obtaining, by the first cognitive system, a back-up frequency in an overlap coverage area, of the second cognitive system having overlap coverage in a target area, transmitted by a terminal; and selecting, by the first cognitive system, at least one frequency from radio communication spectrum resources as a back-up frequency of the first cognitive system in the target area, according to the back-up frequency of the second cognitive system in the overlap coverage area, wherein the selected back-up frequency is not the back-up frequency of the second cognitive system in the overlap coverage area; and wherein the first cognitive system obtaining, the back-up frequency in the overlap coverage area, of the second cognitive system having the overlap coverage in the target area, transmitted by the terminal, comprises: the first cognitive system receiving back-up frequencies of at least one cognitive systems other than the first cognitive system, and coverage areas corresponding to the back-up frequencies of the at least one cognitive systems, transmitted by the terminal; the first cognitive system determining one of the cognitive systems with a coverage area overlapping with the target area as the second cognitive system; and the first cognitive system obtaining the back-up frequency of the second cognitive system in the overlap coverage area; or the first cognitive system obtaining the back-up frequency of the second cognitive system in the overlap coverage area transmitted by the terminal and an instance of time when the terminal lastly obtains the back-up frequency of the second cognitive system in the overlap coverage area; and if back-up frequencies of the same second cognitive system in the overlap coverage area transmitted by at least two terminals are obtained, then the first cognitive system selecting one of the back-up frequencies lastly obtained the terminals at a latest one of instances of time as the back-up frequency of the second cognitive system in the overlap coverage area. obtaining, by the first cognitive system, a back-up frequency in an overlap coverage area, of the second cognitive system having overlap coverage in a target area, transmitted by a terminal; and selecting, by the first cognitive system, at least one frequency from radio communication spectrum resources as a back-up frequency of the first cognitive system in the target area, according to the back-up frequency of the second cognitive system in the overlap coverage area, wherein the selected back-up frequency is not the back-up frequency of the second cognitive system in the overlap coverage area; and wherein the first cognitive system obtaining, the back-up frequency in the overlap coverage area, of the second cognitive system having the overlap coverage in the target area, transmitted by the terminal, comprises: the first cognitive system receiving back-up frequencies of at least one cognitive systems other than the first cognitive system, and coverage areas corresponding to the back-up frequencies of the at least one cognitive systems, transmitted by the terminal; the first cognitive system determining one of the cognitive systems with a coverage area overlapping with the target area as the second cognitive system; and the first cognitive system obtaining the back-up frequency of the second cognitive system in the overlap coverage area; or the first cognitive system obtaining the back-up frequency of the second cognitive system in the overlap coverage area transmitted by the terminal and an instance of time when the terminal lastly obtains the back-up frequency of the second cognitive system in the overlap coverage area; and if back-up frequencies of the same second cognitive system in the overlap coverage area transmitted by at least two terminals are obtained, then the first cognitive system selecting one of the back-up frequencies lastly obtained the terminals at a latest one of instances of time as the back-up frequency of the second cognitive system in the overlap coverage area. the first cognitive system receiving back-up frequencies of at least one cognitive systems other than the first cognitive system, and coverage areas corresponding to the back-up frequencies of the at least one cognitive systems, transmitted by the terminal; the first cognitive system determining one of the cognitive systems with a coverage area overlapping with the target area as the second cognitive system; and the first cognitive system obtaining the back-up frequency of the second cognitive system in the overlap coverage area; or the first cognitive system obtaining the back-up frequency of the second cognitive system in the overlap coverage area transmitted by the terminal and an instance of time when the terminal lastly obtains the back-up frequency of the second cognitive system in the overlap coverage area; and if back-up frequencies of the same second cognitive system in the overlap coverage area transmitted by at least two terminals are obtained, then the first cognitive system selecting one of the back-up frequencies lastly obtained the terminals at a latest one of instances of time as the back-up frequency of the second cognitive system in the overlap coverage area.