The invention relates to an organic selenium triazine polymer and a preparation method thereof. The structure of the compound is as shown in the description, wherein n is larger than 1, X is alkylamine, and Y is dialkylamino. The preparation method includes the steps that during stirring, alkylamine with the equal molar ratio is added into a triglycidyl isocyanurate solution, then the mixture is heated to a certain temperature, temperature preservation reaction is conducted for a certain period of time, and the pH value of the solution reaches 7-8; a certain molar ratio of dialkylamino is added, the solution is heated to a certain temperature, temperature preservation reaction is conducted for a certain period of time, pH value of the solution reaches 7-8, after cooling, suction filtration with solvents recycled, washing, drying and smashing are conducted, and the product organic selenium triazine polymer is obtained. The organic selenium triazine polymer is a good flame-retardant agent, has the char forming and anti-dripping functions, and is good in compatibility with high polymer materials, good in heat resistance and water resistance, excellent in processing performance and capable of being used as flame-retardant agents such as polyethylene (PE), polypropylene (PP) and silicon rubber. Raw materials are low in price and easy to obtain, production cost is low, equipment investment is little, the production process is simple, and large-scale production is easy.