The invention belongs to the technical field of high polymer materials, and particularly relates to a preparation method of a branched nylon, a composite film and a preparation method thereof. Caprolactam, a branching agent and desalted water are put into a reactor, and subjected to hydrolysis ring opening reaction, amidation reaction, chain growth reaction and chain termination reaction, and thebranched nylon is prepared, wherein the branching agent is lysine, according to the invention, lysine is adopted as the branching agent in the preparation process, lysine and caprolactam are subjectedto in-situ copolymerization to prepare the branched nylon, gel particles or partial cross-linked structures are not easily generated, and subsequent processing is more convenient. The composite filmis prepared from the branched nylon, the crystallization degree of the branched nylon is much lower than that of common nylon, and under the condition that the production equipment is not changed, good processing performance is brought, the product performance is equivalent to that of a nylon multilayer composite film, the film is more smooth, and the film is not easy to curl.