The invention discloses a preparation method of an efficient 
phosphorus and 
nitrogen intumescent flame retardant and an application of the 
phosphorus and 
nitrogen intumescent flame retardant to polycaprocactam. The preparation method comprises the steps: carrying out 
condensation reaction on p-
phthalaldehyde and 4,4'-methylenedianiline (DDM), and then, carrying out 
addition reaction on the product and DOPO to obtain 
solid nitrogen and 
phosphorus containing 
flame retardant molecules; and then, uniformly mixing the obtained flame retardant with polycaprolactam resin, and carrying out 
melt blending at 220-260 DEG C to obtain a 
halogen-free flame retardant polycaprolactam material. The method is capable of realizing synthesis at one step without separating intermediate products, simple in synthesis process, simple and convenient in aftertreatment, high in efficiency, safe, environment-friendly and convenient to realize industrial 
mass production; and the flame retardant belongs to a 
polymer-type flame retardant, contains a 
carbon source, an acid source and a 
nitrogen source, does not contain halogens and is nontoxic, 
pollution-free, green and environment-friendly, polycaprolactam reaches the UL94V-0 level when the addition amount of the flame retardant is 15%, and the 
mass percentage of residues is 37.8% at 700 DEG C.