Abstract: A flow injection spectrophotometry method for the determination of aldehydes content in vegetable oil was built.The standard substance was adopted by mixed equal-molar nine aldehydes ( formaldehyde, acetaldehyde, n-propanal, n-butanal, n-pentanal, n-hexaldehyde, n-heptaldehyde, n-octanal and n-nonanal) . The optimal condition for aldehydes determination was obtained by adjusting catalysts, temperature, time, solvents, flow rate, and dosage of reagent, which was suitable to the reaction of aldehydes and 2-naphthylhydrazine to produce the 2-naphthalene hydrazone derivatives under the acid action by self-made flow injection equipment with strong fluorescence-emission at 400 nm. As a result, the linear equation of aliphatic aldehydes was y = 26.8x + 42 ( r = 0.997) , with 1.18% ~ 1.89% of relative standard deviation ( RSD) and 92% ~ 113% of recovery rate.The limit of detection ( LOD) for this method was 0.02 mmol/L ( S/N = 3) .The method can be applied successfully to the determination of aliphatic aldehydes in plant oils with much simple way, high speed and accuracy.