The invention discloses a design and construction method of a foundation cushion layer. The foundation cushion layer comprises a plurality of cushions in one-to-one correspondence to composite pile foundations. The method includes: piling and excavating, to be more specific, constructing the composite pile foundations on a pile foundation construction platform, and excavating from the pile foundation construction platform until a foundation surface is reached; excavating a cushion pit at the edge of each composite pile foundation on the foundation surface, and chiseling off part of each composite pile foundation to allow the top end of the composite pile foundation to be located at the bottom of the corresponding cushion pit; performing cushion film laying and filling, to be more specific,laying an anti-seepage geomembrane in each cushion pit, allowing the anti-seepage geomembrane to attach to the inner wall of cushion pit and extend out of the cushion pit to form a skirt at the edgeof the opening of the cushion pit, and filling the cushion pit with gravels and sand; finally laying a building bottom plate. The method which is applicable to the technical field of building projectcomposite foundation processing has the advantages that construction speed is increased evidently, and cost is saved at the same time.