To explore the mechanism of the potassium effect on the biomass gasification process in supercritical water (SCW), formaldehyde, a typical intermediate formed in the process, was used as the feedstock and the experiments were carried out in a temperature range of 400-650 degrees C, a pressure range of 23-29 MPa, and a residence time range of 4-12 s, with KHCO3, K2CO3, KCl, and mixed potassium salts. The results showed that all potassium salts studied decreased the gasification efficiency and the yields of H-2, CO2, and CO of formaldehyde. The inhibition level of gasification efficiency and hydrogen generation influenced by the potassium salts was on the order of mixed potassium salts > KHCO3 > K2CO3 > KCl. At the high temperatures (500-650 degrees C) and long residence times (8-12 s), the negative effects of the potassium salts on gaseous product generation were enhanced. The effects of the potassium salts on the gasification efficiency and hydrogen generation had slight dependence upon the pressure. At the high temperatures (500-650 degrees C), long residence times (8-12 s), and high pressures (25-29 MPa), each salt in mixed potassium salts had synergetic effects on the gaseous generation. Meanwhile, on the basis of the kinetic model, the kinetics analysis for the effects of the potassium salts was carried out. The results showed that the negative effects of the potassium salts on the yields of gaseous products were obtained by mainly hindering the HCHO direct decomposition reaction.