The invention relates to tripolycyanamide hydrobromide and a preparation method thereof. The preparation method comprises the synthetic steps: introducing melamine with a particle size scope of 1-1000 nm into a closed container, then introducing gaseous state hydrogen bromide, reacting at a certain temperature for a certain time, after the reaction, collecting a white solid which is tripolycyanamide hydrobromide. Due to the fact that the raw materials are ultrafine melamine and gaseous state hydrogen bromide, the raw material source is abundant, no solvent is needed, the preparation technology is simple and the cost is lower than the cost of a water-phase method. Prepared tripolycyanamide hydrobromide provided by the invention is good in thermal stability and good in polymer compatibility; the preparation helps to solve the problems about production cost and technology complicatedness of the water-phase method for producing tripolycyanamide hydrobromide; and tripolycyanamide hydrobromide provided by the invention is wide in application scope ( applicable to inflaming retarding of multiple plastic, rubber, fiber and paints) and has good market prospect.