The invention relates to an organic nitrogen-sulfur-containing ester compound taking a carboxylic group as a terminal group and a preparation method thereof. The preparation method comprises the following steps: firstly, taking dicarboxylic acid with a side group containing nitrogen, and pentaerythritol to react to prepare DAPER taking carboxyl as the terminal group; taking organic nitrogen-sulfur-containing dicarboxylic acid, and dibasic alcohol to react to prepare DADA with the carboxyl at one and the carboxylic group at the other end; finally, taking the DAPER and the DADA to react to obtain a final product. According to the preparation method provided by the invention, the organic nitrogen-sulfur-containing ester compound taking the carboxylic group as the terminal group is finally prepared based on the difference of multifunctional group reaction activity of reactants, and the disadvantages that a traditional liquid ester compound is difficult to store and use, the structure is simple so that the usage is single, the environment is polluted by a preparation process and the like are overcome; the final product is a solid-state ester compound with high molecular weight and the terminal group of a molecule is the carboxylic group and internally contains 17.96 weight percent to 19.79 weight percent of nitrogen and sulfur elements; the organic nitrogen-sulfur-containing ester compound has the advantages of high heat stability, wide application and the like.