The invention provides a method for preparing halogen-free flame retardant nylon 6. Dicarboxylic aid melamine salt, organic diamine cyanuric acid salt or melamine, cyanuric acid, acylamide salt, carbon forming agent, carbonization accelerant, caprolactam and water are added into a reactor, are dispersed and are mixed; the mixture is heated to a temperature of between 230 and 265 DEG C, is stirred, is subjected to polymerization reaction for 5 to 8 hours at a pressure of between 0.2 MPa and 0.8 MPa, and is subjected to pressure relief, decompression, vacuumization and low molecule removal to obtain the halogen-free flame retardant nylon 6. The material prepared by the method has good stability and little toxicity, substantially improves mechanical performance, has lasting flame retardance and can be directly used for flame-retardant yarn or engineering plastic.