Abstract: The Carbon fiber reinforced plastics (CFRPs) have gained wide attention due to their excellent properties including light weight and high-strength. Among them, the sandwich structure, due to its diverse functions and strong designability, has been increasingly widely applied. However, the internal damage is prone to occur at the interface between the core material and panel during the service stage, reducing the structural loading capacity. The Fiber Bragg Grating (FBG) arrays embedded inside the large-sized composite sandwich plates form a three-dimensional fiber sensing network. The tests including the pre-impact four-point bending, low-velocity impact, and post-impact four-point bending were carried to achieve the in-situ monitoring of strains at different thicknesses under dynamic and static loads. The peak strains during the impact test were extracted and the impact energies were predicted using the Gaussian process regression based on the particle swarm optimization algorithm. The prediction results were in good agreement with the actual impact energies. After the inside damage occurs, the local stiffness at the interface decreased significantly, and the loading capacity of the sandwich plate decreased. The FBG arrays provide local damage information and compensate the deficiency of the macroscopic load-displacement curve. The three-dimensional fiber sensing network can effectively realize the sensing of the impact process of the composite sandwich structure and the real-time health monitoring of impact damage at the interface between the core material and panel, which provides effective information for ensuring the service safety of the composite sandwich structure.