This article based on OFDMA cognitive radio networks and spectrum allocation issues, In the overlay spectrum sharing (Spectrum Underlay) way, Research the downstream of a single cellular network cognitive radio spectrum resources allocated. We proposed allocation algorithm by jointly considering the cognitive user priority and sub-set with availability the sub-carrier band equalizer, meanwhile based on the lowest transmission needs of cognitive users to achieve a fair distribution of the sub-carrier band between cognitive user. To make better use of spectrum resources.