This paper points to the dynamic spectrum sharing problem in cognitive radio network and analyzes the behavior of primary user (PU) and secondary user (SU). The relationship between PU and SU can be treated as principal and agent. Then, a principal-agent theory based two-step game model is proposed with the consideration of cooperation and price coeffcient which effect the SU's choice and PU's profit. Further, some incomplete information is assumed as random variables so that certainty equivalent is introduced in the model. Both cooperation and price effect are simulated at the end.