Abstract: Modern power system is developing into a cyber physical power system (CPPS). The current differential protection is a typical application with ICT and physical function tightly coupled. So it is necessary to study its reliability from the perspective of CPPS. On the basis of the closed-loop control process of physical-measurement-communication-decision making, the paper analyzes the integrated effects of cyber and physical factors, such as line current, measurement error of CT, communication delay and protection criteria, on the current differential protection, and proposes the mis-operation probability model of the current differential protection. On that basis, a simulation is performed to verify the model, and the key factors affecting the reliability are analyzed.