The invention provides a panel-core reinforced composite grating sandwich plate and a preparation method thereof and belongs to the technical field of composites. The method comprises steps as follows: step one, preparing a composite upper panel provided with slots and a composite lower panel provided with slots by the aid of a panel mold; step two, designing and assembling a grating core movablemold; step three, preparing a composite grating core by the grating core movable mold; step four, preparing the composite grating sandwich plate. The composite grating sandwich plate core has orthotropy, the panels and the core are embedded through the slots, interface bonding capacity is enhanced, bearing capacity of the composite grating sandwich plate is improved, besides, a core mold with multiple rank combinations can be assembled by the grating core movable mold by means of free disassembling combination of movable blocks, so that the homogenous unit-cell composite grating sandwich plates with different arrangements and combinations can be prepared, the technology is simple, and demoulding is easy.