Disclosed are a method and device for determining and assisting in determining a back-up frequency in a cognitive radio system. A first cognitive system shares the wireless communication spectrum resources of an incumbent system, and the method thereof for determining a back-up frequency comprises: through a terminal, a first cognitive system obtaining a back-up frequency of a second cognitive system which has overlapping coverage in a target area in an overlapping coverage area; and according to the back-up frequency of the second cognitive system in the overlapping coverage area, selecting from the wireless communication spectrum resources at least one frequency as a back-up frequency in the target area. When determining the back-up frequency, the first cognitive system can refer to the back-up frequency of the second cognitive system to conduct determination, thereby optimizing the system performance.