The invention discloses a preparation method for a reaction type nitrogen-phosphorus flame retardant and relates to the technical field of a nitrogen-phosphorus flame retardant and a preparation method thereof. The method comprises the following steps: 1) purifying pentaerythritol phosphonate (PEPA); 2) activating PEPA with isocyanic acid; and 3) preparing the reaction type nitrogen-phosphorus flame retardant: weighting unsaturated aliphatic amine (alcohol), dropwise adding into the PEPA activated with isocyanic acid in the step 2, reacting for 4-10h at the revolving speed of 80-230r/min in a water bath kettle at 40-120 DEG C, ending the reaction, washing with a xylene solvent for 2-4 times, and performing vacuum drying for 10-72h at 70-120 DEG C, thereby acquiring the reaction type nitrogen-phosphorus flame retardant. The fire retardant disclosed by the invention is mainly composed of nitrogen, phosphorus and carbon elements, has excellent thermal stability and chemical stability, is halogen-free, low-smoke, low-toxicity and environment-friendly, contains double active bonds, can be directly used as a chain extender for reacting with a monomer for compounding an anti-flaming macromolecule material, and is simple in bonding mode.