Abstract: To address the difficulty of conventional methods in the effective exploitation of unconventional geological resources and energy, as well as the increasingly acute environmental conflicts, an in-situ coal pyrolysis method employing supercritical water and superheated steam was adopted to convert solid coal into combustible gas. Using a newly developed high-temperature and high-pressure reactor, pyrolysis experiments were conducted on lump bituminous coal. The mechanisms by which supercritical water and superheated steam affect coal pyrolysis were investigated, and the influence of temperature and pressure under both conditions on the pyrolysis products and pyrolysis rate of coal was analyzed.The results showed that, under the action of supercritical water, the gas yield at 700 ℃ was approximately 43 times higher than that at 400 ℃. At 400–500 ℃, the gas production rate continuously decreased with time, exhibiting a logarithmic relationship. When the temperature rose to 600–700 ℃, no reaction termination or rate deceleration occurred within 24 h; the two showed a linear relationship, and the coal continuously released gas at a stable rate. In the released gas, the H₂ content increased by 29%, the CH₄ content decreased by 7%, the CO and CO₂ contents decreased by 10% and 12% respectively, and the weight loss rate increased by 20%.Under the action of water, changes in pressure had little effect on the coal pyrolysis reaction. Only after reaching the critical point (temperature = 374 ℃, pressure = 22.1 MPa) did the gas yield under supercritical water exceed that under superheated steam by about 16%. Compared with the superheated steam stage, the gas components produced in the supercritical water stage were dominated by H₂, with reduced CO₂ and CO contents and essentially unchanged CH₄ content. Specifically, the H₂ content in the released gas increased by 14%, the CO and CO₂ contents decreased by 3% and 13% respectively, and the weight loss rate increased by 3%. The research findings provide important reference value for determining the process parameters of in-situ coal fluidized mining.