The invention discloses a composite-material core-level cover plate for a rocket, and a preparation method thereof. The cover plate comprises nine detachable units, wherein each detachable unit is a composite-material interlayer structure containing an inner framework; and one is a central quadrilateral unit and the other eight detachable units are divided into four groups, the two detachable units in each group are connected together through a hinge, the four sides of the central quadrilateral unit are lapped and fixed to one turned edge of the four groups of detachable units separately, theother sides of the four groups of detachable units are placed on a launching platform installation frame and fixed, and the whole outer edge outline is determined by an installation connector space where the cover plate is located. The invention puts forward for the first time that a light-weight foam interlayer structure containing the inner framework is applied to the core-level cover plate forthe rocket. The composite-material core-level cover plate has the advantages of being convenient to transport, simple to install, rapid to detach, and the like; and the disclosed preparation method has the advantages of ensuring the good forming quality of products, realizing integrated forming, and achieving low cost.