Aiming at guarding high-orbit high-value targets, a multi-spacecraft cooperative guard strategy based on reachable domain coverage was proposed. The cooperative guard mission was described from the perspective of relative motion, and the multi-pulse reachable domain of the threat was modeled as a convex optimization problem. In the framework of receding optimization, the guard planes and points were designed based on the dynamically updated terminal reachable domain of threat, and a multi-spacecraft cooperative trajectory planning model was constructed with the guard points as terminal position constraints, the corresponding guard trajectories were generated. Simulation results show that the proposed method can quickly calculate the terminal reachable domain of the threat. The cooperative guard strategy can effectively prevent the threat in multiple scenarios, and the guard success rate increases with the enhancement of the maneuvering ability of the guard spacecraft.