Abstract: The aquatic feed sample (about 0.2 g) was added into 10 mL of 250 g·L^-1 potassium hydroxide-methanol solution. The mixture was heated at 60 ℃ for 6 h, vortex oscillated every 2 h, then centrifuged at 3 000 r·min^-1 for 15 min. 0.10 mL of the supernatant was added into a 40 mL brown sample bottle with 34-38 mL of water added in advance, and the pH of the solution was adjusted to 6-8 with HCl (1+9) solution. 0.3 mL of 2 mol·L^-1 sodium acetate buffer solution and 0.05 mL of ethylation reagent were added to carry out derivatization. Then the solution was made up to 40.0 mL with water. After mixing well, purge and trap-gas chromatography-atomic fluorescence spectrometer was used for determination. The results showed that the fluorescence intensity was kept linear with the mass of methylmercury within 500 pg. The detection limit (3s/k) of the method was 2 μg·kg^-1, and the lower limit of determination (10s/k) was 5 μg·kg^-1. Values of recovery obtained by standard addition method were between 94.7% and 98.8%, and the relative standard deviation (n=6) of determined values was 4.9%.