Abstract: For the application requirement of radar and communication integration, this paper puts forward a method of spectrum sharing based on mutual information. First of all, establish the signal model of radar and communication integration system. Then, the evaluation index of radar performance based on mutual information theory is constructed. On this basis, the performance evaluation mode of radar and communication integration system based on information theory is established by taking communication rate as the performance index of communication. Next, the spectrum allocation mode is divided into traditional spectrum non-sharing allocation mode and two spectrum sharing mode according to the overlap degree of received radar signal and communication signal. And the inner bounds of performance of radar and communication integration system for three spectrum allocation modes are derived. Finally, the effectiveness of the proposed method is verified by simulation．