Abstract: In most of the existing works on cognitive radio (CR)systems,the multi-user OFDMA system how to realize the reasonable allocation of sub-carriers and power is one of the key.In this paper, we aim at the process of radio resource allocation in cognitive multi-user resource allocation unfair question,when sub-carrier is occupied by the authorized user,cognitive user throughput will be restricted.We propose an optimization algorithm based on spectrum underlay joint sub-carrier and power allocation in OFDMA system.The algorithm set interference threshold and use the raw sensing information(RSI) and the channel state information (CSI) for power and sub-carrier distribution,then calculate the power control and user selection,At last,find the optimal transmission power and optimal user for each sub carrier,which can ensure the authorized users away from harmful interference,so that the presence of an authorized user can also share the spectrum transmission and improve the network throughput.Theoretical analysis and simulation results show that our algorithm enhances average weighted throughput,compared with traditional algorithm of sub-carrier and power allocation.