Abstract: Several large-scale oversea blackouts in recent years have shown that coordinated attacks on power cyber-physical system can lead to the aggravation and rapid spread of grid failures, and even system collapse. Focusing on the emergency that system faults can not be eliminated in a short time and has a tendency of rapid spread, this paper adopted the idea of saturation defense, and proposed a novel method of cyber-physical system saturation defense based on active cut set. The algorithm synergistically considered the structure, resource and operation modes of the cyber subdomain and the physical subdomain, and used the improved weighted GN split algorithm to search for the feasible domain of cyber cut surface. Then, through the constraint of coherent groups, power balance and the minimum impact calculation of active power flow, a feasible scheme for partition was optimized. Therefore, while ensuring the minimum loss of cyber network communication performance and the stability constraints of physical system, a unified cut surface was found. Active segmentation of threat sub-zone was realized with the smallest performance loss. The expansion of attack damage was prevented. The stable operation of security main zone was maintained. Finally, an improved IEEE39 bus cyber-physical system was used as a simulation. It was verified that the proposed method has an obvious ability of attack blocking and the superiority of low lost of load probability under different attack intensities.