Abstract: The influences of temperature, time, and mineral matter on conversion of lignite in a semi-continuous supercritical water reactor (SCWR) were investigated. The evolution of pore during reaction in SCWR was deduced with Fourier transform infrared and Raman spectra characterization. It is found that supercritical water can quickly extract the volatile from coal under low temperature, which promotes char graphitization and formation of carbon precursor. When temperature is above 550 ℃, more C-O-C cross-linking structures are formed, accompanied by a significant increase of surface area. The extraction yield of deashed coal is relatively high during pyrolysis process and more micropores are formed compared with raw coal. Furthermore, mineral matter in coal promotes the formation of mesopore.