The invention relates to the technical field of synthesis of a flame retardant and provides a sulfonate flame retardant with high nitrogen and phosphorus content. The sulfonate flame retardant comprises a compound I with a structure shown in the specification. The invention further provides a preparation method of the sulfonate flame retardant with high nitrogen and phosphorus content. According to the preparation method, cyanuric chloride, triphenylphosphine oxide, sodium sulfanilate, concentrated nitric acid and concentrated sulfuric acid are taken as raw materials, and the compound I is synthesized through four steps of reactions. The invention further provides an application of the sulfonate flame retardant with high nitrogen and phosphorus content in addition to polycarbonate. The sulfonate flame retardant, the preparation method and the application have the beneficial effects that the flame retardant is non-toxic, smoke-suppressing, droplet-resisting, good in thermal stability, high in flame retardant efficiency and low in cost; the usability and color of a product are not affected when the sulfonate flame retardant is added to the material, a nitrogen and phosphorus synergistic flame retardant effect can be realized, and a synthesis process is simple and suitable for popularization and application.