The work aims to synthesize melamine cyanurate (MCA)in a new method, and explore its application in polyamide 6 (PA6). Melamine (MA), cyanuric acid (CA) and a small amount of water were mixed into paste and the paste reacted for a certain time at room temperature. Then, a small amount of MCA was added for the paste to continue reacting for the preparation of MCA (flame retardant).The MCA and PA6 were blended to prepare flame retardant PA6 composites. MCA was characterized by FTIR, XRD, TG and SEM, and the flame retardancy and mechanical properties of flame retardant PA6 composites were tested. The results showed that the FTIR and XRD characteristic peaks of MCA were consistent with those of the MCA synthesized in the water. The maximum thermogravimetic temperature of the synthesized MCA reached 451.7 °C. When the mass fraction of flame retardant was 8%, the limited oxygen index (LOI) of flame retardant PA6 composites reached 29% and the flame retardancy was UL-94 V0. With increase of the flame retardant content, the mechanical properties of the composites decreased gradually. When the mass fraction of flame retardant was 8%, the tensile strength was 66.4 MPa and the impact strength was 4.7 kJ/m2. Therefore, the MCA synthesized by the proposed method and featured by simple technology, requiring no heating and low water consumption, greatly improves the flame retardancy of PA6 composites.