The invention discloses a method for synthesizing a melamine cyanurate fire retardant by a solid-phase synthesis method. A grinding device is needed; the grinding device comprises a machine frame, wherein an arc-shaped through hole is formed in the machine frame; a first rolling shaft is glidingly connected in the arc-shaped through hole; a grinding roller is rotationally connected onto the first rolling shaft; a connecting rod is hinged onto the first rolling shaft; a reciprocating movement device is hinged onto the connecting rod; a grinding seat is abutted against the lower part of the grinding roller; a grinding groove matched with the grinding roller is formed in the grinding seat; one end of the grinding seat is hinged to a first air cylinder; a strip-shaped hole is formed in the other end of the grinding seat; a second rolling shaft is glidingly connected in the strip-shaped hole; a second air cylinder is hinged onto the second rolling shaft. The method comprises the following steps of (1) mixing; (2) transfer reaction; (3) water washing; (4) centrifugation; (5) drying. Compared with the prior art, the method has the advantages that by simplifying the steps and reducing the equipment use, the operation difficulty is lowered; the product production can be completed without water; the water resources are saved; the energy consumption is reduced.