The invention relates to an acidic-group-terminated organic phosphorus-containing ester and a preparation method thereof, and a flame-retardant polyamide 6 fabric prepared from the ester. The method comprises the following steps: carrying out reaction on side-group phosphorus-containing dicarboxylic acid and pentaerythritol to obtain carboxy-terminated DAPER, carrying out reaction on organic phosphorus-containing dibasic acid and dibasic alcohol to obtain DADA of which one terminal is an acidic group and the other terminal is hydroxy, and finally, carrying out reaction on the DAPER and DADA to obtain the final product. The prepared acidic-group-terminated organic phosphorus-containing ester is the high-molecular-weight solid ester, and the terminal group of the molecule is carboxy or phospho; and the acidic-group-terminated organic phosphorus-containing ester contains 9.66-13.69 wt% of phosphorus, and has the advantages of high heat stability, wide applicability and the like. The final product can be used as a flame retardant for finishing a polyamide 6 fabric by the following steps: pretreating the polyamide 6 fabric, carrying out hydroxymethyl modification, and carrying out flame-retardant finishing by a double-dipping double-rolling technique, thereby obtaining the flame-retardant polyamide 6 fabric.