To improve the system fairness of Spectrum Allocation on cognitive radio system, this paper presents a spectrum allocation algorithm of combining two standards on cognitive radio system. Firstly, some subgraphs allocate spectrum by using the Collaborative Max Sum Bandwidth (CMSB) standard, after finishing spectrum allocation, then other subgraphs adopt Collaborative Max Proportional Fair (CMPF) standards. Compared with the previous improved parallel algorithm, the simulation shows that the system fairness has been improved obviously, at the same time, the channel capacities of bottleneck cognitive users have been improved greatly.