The invention discloses a resource allocation method for a cognitive system based on space multiplexing. In the method, the F norm of a channel matrix is used to select a cognitive user so as to obtain a multi-user diversity gain of a cognitive network, a double-selection mode is used to lower the complexity of the algorithm, transmitted signals of the cognitive users are projected to a null space of an interference channel so as to avoid the interference of the cognitive user on an authorized user, the channel matrix of the cognitive user is converted into a concurrent independent channel byusing a singular-value decomposition method, and the system capacity is enlarged by using a water-filling power allocation method. In a word, by using the multi-user diversity gain of the cognitive network, the method simplifies the user selection method and improves the system performance under the condition of acceptable computation complexity.