Aiming at the problem that the traditional power constraint can not guarantee the delay of the main users in the cognitive radio system, this paper proposes a new effective user capacity constraint model, which makes the cognitive radio system meet the needs of the cognitive spectrum sharing, So that the main users in the delay under the constraints of stable transmission. Based on this model, the lower bound performance of cognitive user ergodic capacity in Rayleigh fading environment is deduced. The numerical simulation results show that the proposed constraint model can guarantee the transmission capacity of the cognitive user while transmitting the main users.