This paper characterizes the inter-base station interference of cooperative integrated sensing and communication (ISAC), which is considered as one of the key technologies in the sixth-generation mobile communication system (6G). First, a cooperative ISAC cellular system model is presented. Then, the inter-base station interference is analyzed and evaluated for two types of sensing targets. Based on the evaluation results, the principle for the interference management due with the inter-base station interference that suits for high frequency and low frequency systems is respectively proposed to ensure high detection probability and improve sensing accuracy. A ring networking scheme is proposed to further improve the sensing accuracy. The performances are verified through simulations.