Abstract: Using pig dung as material, the effect of formalin on biogas fermentation were studied in this paper. The experimental results showed that the formalin disinfectant had restraint effect on biogas fermentation. After adding formalin, the content of total volatile acid and lipase activity increased, but amylase activity, cellulase activity and gas yield decreased. With increasing of dosage, the change became strengthening. When the addition of formalin reached to 0.05%, the content of total volatile acid increased by 18.78%, lipase activity increased by 135.13%, and amylase activity and lipase activity decreased by 78.57% and 83.65%, respectively. The results of research explained why gas yield decreased when the faeces of livestock farms was handled with for biogas fermentation. The results can provide a certain theoretic reference for biogas fermentation of faece from livestock farms.