The pyrolytic behavior of citric acid was investigated by thermogravimetric system coupled to Fourier transform infrared spectrometer (TG-FTIR). The TG curves, DTG curves, and relative contents of pyrolytic gas phase products of citric acid in oxygen of different concentrations were determined and compared. The formation rules of gas phase products and pyrolytic mechanism of citric acid were explored. The results showed that: 1) The main pyrolytic gas phase products of citric acid included CO₂, H₂O, CO, anhydride and ketone compounds, in which CO₂ was the major, while CO was the minor, H₂O emerged first, while anhydride the last. 2) There might exist two pyrolytic pathways, in one of the pathways citric acid was dehydrated and then decarbonated to form anhydride, and in the other to form ketones mainly by decarbonation. At the initial pyrolytic stage, both pathways coexisted, increasing oxygen concentration would benefit the formation of ketones. While at higher temperature, anhydride formation dominated.