Abstract:The sample was acidified after adding hydrochloric acid-sodium chloride solution，then extracted by Ultrasound-assisted. The sodium acetate buffer and 2， 4-dinitrobenzene hydrazine derivative liquid were added in extraction solution， had derivatization by water bath at 50 ℃， it was detected after filtered. At the same time， the formaldehyde content in the fish fillets was detected. The content of hydroxymethanesulfinic acid monosodium salt dihydrate was the difference between the measured value of hydroxymethanesulfinic acid monosodium salt dihydrate and formaldehyde. The result showed that the method was a good linear， r2=0.999 97. The recoveries was between 92.3 % and 102 %.