The invention relates to a terminal-hydroxyl organic phosphorus-nitrogen ester content and a preparation method thereof and flame-retardant polyester fiber FDY (fully drawn yarn) made form the terminal-hydroxyl organic phosphorus-nitrogen ester content. Firstly, DAPER with terminal being hydroxyl is obtained through reaction of lateral-group functional element contained binary carboxylic acid with pentaerythritol, then, DADA with the terminal being the hydroxyl is obtained through reaction of organic functional element contained binary acid with dihydric alcohol, and finally, a final product is obtained through reaction of the DAPER and the DADA. The final product is of the solid ester content with high molecular weight, the terminal of the molecular is the hydroxyl, 8.48-15.53wt% of phosphorus and nitrogen are contained, and the ester content has the advantages of high thermal stability, wide application and the like; the final product is taken as the a flame retardant to prepare the flame-retardant polyester fiber FDY, the limit oxygen index of the finally prepared flame-retardant polyester fiber FDY is greater than or equal to 35%, filament number is from 3.0dtex to 5.0dtex, and breaking strength is greater than or equal to 3.8cN/dtex.