The invention relates to a method for preparing nitrogen phosphate sulfur-containing reactive flame retardant applied to epoxy resin. The preparation method comprises the following steps: sequentially adding p-hydroxy benzaldehyde, aminothiazole and a solvent into a reactor which is provided with a condensing pipe and a stirrer, and introducing inert gas to perform first reaction; adding 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to carry out second reaction; and pumping, filtering, washing and drying the system to obtain white or light brown nitrogen phosphate sulfur-containing reactive flame retardant after the reaction is completed. The method has the advantages that 1, the synthesizing process is short, the cycle is short, post-treatment is easy, and control and industrial production are easy; 2, the prepared flame retardant contains nitrogen, phosphate and sulfur three flame-retardant elements at the same time, has a synergistic flame-retardant effect, and can be used for remarkably improving the flame retarding performance of epoxy resin; and 3, the prepared flame retardant is a reactive flame retardant, and can be used for keeping stability and mechanical performance of cured epoxy resin while improving the flame retarding effect of the cured epoxy resin.