A method of gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) to test stable carbon isotope ratio offormaldehyde(FA) in squid was established. After the squid samples were fully extracted with water， the oil was removed by centrifugation at high speed. The supernatant was derivatized with 2， 4-dinitrophenylhydrazine at 60 ℃ for 1 h. The derived liquid was extracted with n-hexane liquid. Dichloromethane， methanol， n-hexane and acetonitrile can be used as solvents for isotopic testing of formaldehyde 2，4 dinitrophenylhydrazone. No carbon isotope fractionation occurred in the derivative reaction of formaldehyde. The standard deviation of δ13C value of formaldehyde 2，4-dinitrophenylhydrazone from measured value and calculatedvalue by the mass balance equation was 0.215‰. The formaldehyde content of squids from 9 cities was 0.341-39.132 mg/kg， and the content in visceral was higher. The difference value of δ13C value in sample 2， 3， 4 and 5 with high formaldehyde content was as high as 23.653‰. The fingerprint of formaldehyde stable carbon isotope in aquatic products can be used as the characteristic index of source analysis.