Cyber-physical system (CPS) is a next-generation intelligent system integrating computing, communication, and control. As a unit of computing process and physical process, CPS is a new research field, where cooperative adaptive cruise control (CACC) is not only a microcosm of CPS but also a prerequisite for unmanned systems. CACC relies on the wireless communication network technology to achieve cooperative platooning control through vehicle-to-vehicle collaborative control methods. It can improve traffic efficiency and ensure safe driving. Once the wireless network is introduced by each vehicle to exchange information, it is vulnerable to cyber attacks, making attack detection necessary. In this paper, an integral sliding mode observer is designed to monitor malicious attacks. The proposed sliding mode observer not only retains the performance of traditional sliding mode control but also realizes the finite-time observation, avoiding the singular phenomenon that may occur in the traditional terminal sliding mode.