The invention discloses a polyamide intumescent flame-retardant char-forming agent having a main chain containing a piperazine structure; the char-forming agent has the structure general formula defined in the specification, wherein R1 is any one of H, straight-chain or branched-chain alkyl with the number of carbon atoms of 1-4, phenyl, benzyl or phenol, R2 represents straight-chain or branched-chain alkyl with the number of carbon atoms of 1-8, and n is more than or equal to 1. The invention also relates to a preparation method of the intumescent flame-retardant char-forming agent, an application of the intumescent flame-retardant char-forming agent as a carbon source of a novel intumescent flame-retardant system, and an application of the novel intumescent flame-retardant system in halogen-free flame retardancy of polyolefin, polyurethane, polylactic acid, polyamide, ethylene-propylene-diene monomer, silicone rubber and epoxy resin. The prepared intumescent flame-retardant char-forming agent has the advantages of no halogen, low smoke, low toxicity, good thermal stability, high char-forming efficiency, strong water resistance, relatively little influence on mechanical properties of flame-retardant polymers, and simple synthesis process, can be prepared by a one-step method, and is prone to industrialized production.