step (1), hydrolyzing a loach protein with proteases to obtain an enzymatic hydrolysis product;

step (2), separating the enzymatic hydrolysis product using an ultrafiltration membrane, and taking a component with a molecular weight less than 3 kilodalton (kDa); and

step (3), eluting the component with the molecular weight less than 3 kDa by using deionized water with an elution flow rate of 0.8 milliliters per minute (mL/min) by Sephadex G-15, and taking an eluent with a retention time of 125-155 minutes to thereby obtain the oligopeptide;

wherein in the step (1), the loach protein is hydrolyzed using the bromelain and the alkaline protease sequentially;

wherein a hydrolysis time using the bromelain in the step (1) is in a range of 4-6 hours, and a hydrolysis time using the alkaline protease in the step (1) is in a range of 0.5-1.5 hours; and

wherein the amino acid sequence of the oligopeptide with lipid-lowering activity is SEQ ID NO: 1.