Abstract: The influences of potassium salts on the decomposition of formaldehyde in supercritical water were investigated in a continuous reactor under 400～650℃, 23～29 MPa and a residence time of 4～12 s, as formaldehyde is one of the most important intermediate products for the gasification of biomass with supercritical water. The results showed that KHCO₃, K₂CO₃, KCl and mixed potassium salts are able to reduce the fraction CO in the gaseous product and increase the fraction of CO₂, which then depresses the heating value of the gaseous product. All these potassium ingredients exhibit an inhibitive effect on the formation of H₂, CO and CO₂ and the gasification efficiency; the inhibition strength of various potassium ingredients follows the order of mixed potassium salts > KHCO₃ > K₂CO₃ > KCl. The inhibitive effect on the formation of gaseous products is enhanced under high reaction temperature and long residence time, but is almost independent on the reaction pressure. Under high temperature, high pressure and long residence time, gas generation may be inhibited considerably by the mixed potassium salts, possibly due to a synergetic effect of different potassium salts.