Influence of different bombayduck (Harpadon nehereus) organs on the formaldehyde content of muscle was studied. Kidney was proved to have the highest formaldehyde producing activity among the organs. An extract was obtained from the kidney. Trimethylamine-N-oxide can be decomposed to formaldehyde and dimethylamine by the extract. The optimum catalyzing conditions of the extract were found to be 50℃，pH=7 by the responding surface method(RSM), and it can be inactivated by the trichloroacetic acid to lose the ability of decomposing trimethylamine-N-oxide. The extract shows typical enzyme properties in different temperature and pH value. There was a stable ratio between formaldehyde and dimethylamine which ranged from 1.6-1.8. All above explain why there was formaldehyde in the bombayduck and proved that some formaldehyde was endogenous.