Supercritical water gasification (SCWG) is an innovative handling method for black liquor, which can eliminate its pollution and produce hydrogen simultaneously. In this study, we investigated SCWG of wheat straw soda black liquor with higher temperature and longer reaction time for efficient gasification in a batch reactor. The influence of temperature (600–750 °C), reaction time (10–50 min), and black liquor concentration (2.5–9.5 wt %) were studied. Higher temperature, longer reaction time, and lower concentration promoted hydrogen production and carbon conversion. For black liquor with an initial concentration of 9.5 wt %, maximum carbon conversion of 94.10% was achieved at 750 °C. Higher carbon conversion (98.17%) was obtained when black liquor was diluted to 2.5 wt %. From thermodynamic analysis and experimental results in literature, we compared low- and high-temperature SCWG of black liquor and found that high-temperature SCWG has better opportunities. On the basis of gasification results, potentially 0.042, 1.12, and 12.00 million tons of hydrogen are estimated to be produced annually from SCWG of black liquor in a typical pulp mill, China, and the world, respectively. Producing so much hydrogen by SCWG of black liquor instead of natural gas reforming can reduce roughly 0.51, 13.63, and 146.54 million tons of CO2 emission annually. More CO2 emission can be reduced when SCWG is used to replace coal gasification. The results suggest SCWG of black liquor can make a great contribution to solving the energy shortage and global warming problems.