Toxic formaldehyde is sometimes used illegally as a food preservative, however, on-site rapid analysis of trace formaldehyde in aquatic products remains a challenge. In this work, a simple on-site rapid quantification method for trace volatile formaldehyde in aquatic products was developed by a derivative reaction-based surface enhanced Raman spectroscopy (SERS) technique coupled with a homemade portable purge-sampling device. Trace formaldehyde separated from complicated aquatic matrices via a purge-sampling procedure was reacted with a derivative reagent to produce a Raman-active analyte for consequent SERS analysis. Au/SiO2 nanoparticles (NPs) were employed as the enhancement substrate to achieve significant enhancement of Raman signal intensity. Conditions of derivative reaction and SERS detection were optimized in detail, and the selectivity of this analytical method was also evaluated based on related analogs. Under optimal conditions, an extremely low detection limit of 0.17 μg L(-1) was achieved. Trace volatile formaldehyde can be found in fresh squid and shrimp samples without obvious matrix interference, and this was quantified to be 0.13-0.21 mg kg(-1) using the described method. The recoveries of spiked aquatic product samples were found to be 70.0-89.1% with RSDs of 2.3-7.2% (n = 3). The results suggest that the proposed method is reliable and suitable for on-site rapid analysis of trace formaldehyde in aquatic products.