The invention discloses a preparation method of a phosphorus-nitrogen intumescent fire retardant containing a Schiff-base derivative, and an application of the fire retardant in polycaprolactam. The method comprises performing a condensation reaction of terephthalaldehyde and diethylenetriamine; then performing an addition reaction with DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) to obtain nitrogen-phosphorus contained solid fire retardant molecules; uniformly mixing the obtained fire retardant with polycaprolactam resin; performing melt blending at a temperature in a range of 220-260 DEG C to obtain a halogen-free fire retardant polycaprolactam material. In the method, synthesis is performed by a one-step method, so that separation of intermediate products is not needed; and the processing steps are simplified, and the method is high-efficiency, safe and environmentally friendly, and is convenient for industrialized batch production. The fire retardant belongs to polymer-type fire retardants, has a carbon resource, an acid resource and a nitrogen resource, has no halogen, has no toxicity and no pollution, and is green and environmentally friendly.