In order to mitigate the propagation of inter-area cascading failures, this paper proposed a novel soft isolation method. Based on the recently developed Interaction Graph (IG) approach, a novel KICs-based OPF model, which considered the status of KICs, is proposed to realizing soft isolation. By intentionally limiting the disturbance on the tie lines between sub-grids, the proposed soft isolation control strategy can significantly mitigate the propagation of inter-area cascading failures and give special protection of the main grid. The Numerical results verified the efficiency of the proposed method.