Under the constraints of transmission power of the cognitive users,the optimization process of cognitive users’utility was analyzed with Rayleigh Fading model,and then an algorithm based on water-level continually adjustment was proposed to solve the problem of power allocation and communication utility computation. This algorithm was also applied to spectrum allocation,so that a cognitive radio spectrum allocation mechanism based on channel bisection was proposed too. From this,the users’utility under different optimization models could be obtained. Simulation results show that the model based on proportional fairness not only provide fair spectrum allocation,but also has a higher total utility.