Black liquor is a major wastewater generated from the pulping process that has a detrimental impact on the environment. This work assessed the potential of black liquor to be an inexpensive resource of alkali catalyst in supercritical water gasification of coal through thermodynamic analysis and experimental study. The experiments were performed in a fluidized-bed reactor at 550 °C and 25 MPa, and the products were characterized by gas chromatography, X-ray fluorescence, X-ray diffraction, and gas chromatography-mass spectrometry. In the gasification of coal and black liquor mixtures, the presence of coal can improve the H2 production under the equilibrium state. Both the inherent alkalis and lignin in black liquor played a role of improving the gasification efficiency of coal. The alkalis also accelerated the water–gas shift reaction and increased the H2 fraction. The high total mixture concentration inhibited the gasification, and the reactor was plugged with a concentration of 25 wt %. The presence of black liquor fixed more sulfur in the solid residues, but it aggravated the corrosion of the 316 SS reactor. The aqueous product mainly contained alkylphenols, cyclopentanone, and their derivatives. A simple influencing mechanism of lignin on coal gasification was proposed: the decomposition of lignin prior to coal can generate some phenolic compounds, which can promote the extraction of the substances coated outside the coal particle and favor the further reaction between the coal and water.