Abstract: With the large-scale integration of renewable energy into power grids, the demand for flexible regulation of coal-fired power units is increasing. Complementary operation between solar energy and coal-fired units is considered an effective approach to enhancing system flexibility and reducing carbon emissions. Meanwhile, biomass, as another form of green renewable energy, can be co-fired or thermochemically coupled with coal units to partially replace coal and further reduce carbon emissions. Based on this concept, a novel system integrating a coal-fired unit with solar-driven biomass supercritical water gasification （SCWG） is proposed. In this system, a portion of the main steam from the coal-fired boiler is extracted to participate in the solar-driven SCWG process, achieving synergistic coupling among coal, solar, and biomass energy sources. During gasification, biomass reacts with high-temperature and high-pressure water to produce CH₄, H₂, and CO gases, which are subsequently converted into CH₄ through a methanation process and stored in a gas tank. When the grid load fluctuates, the stored methane can be rapidly fed into a gas turbine for power generation, leveraging its high ramping rate to compensate for the slower response of the coal unit. This forms an integrated “electricity–heat–gas” multi-energy complementary regulation mechanism. A solar–biomass supercritical water gasification coupled coal-fired power plant system is established based on Aspen HYSYS. The effects of key parameters such as gasification temperature and water–carbon ratio （the molar ratio of water to carbon in biomass） on the thermodynamic performance of the supercritical water gasification process are analyzed. The system’s overall performance—energy conversion efficiency, specific coal consumption, and ramping capability—was evaluated under typical operating conditions. Results show that at 50% load, a gasification temperature of 700 °C, and a steam-to-carbon ratio of 1.3, the total energy conversion efficiency reaches 71.27%, with methane accounting for 56.03% of the total energy output as the primary energy carrier, and net electricity generation accounting for 15.23%. Across the 20%–75% load range, the system efficiency remains stable between 61.29% and 73.75%, indicating excellent stability and flexibility in the coordinated operation of solar and coal-fired units. Compared with conventional coal-fired units, the proposed system exhibits significant potential for coal consumption reduction under deep peak-shaving conditions. The specific coal consumption of a traditional unit rises to 311.49 g/kWh at 30% load, whereas the coupled system achieves 242.85 g/kWh at 20% load, showing a marked improvement in efficiency. The system employs a Mitsubishi 701F gas turbine （240 MW）, which has the potential to enhance the overall system ramp rate to 15.3 MW/min. The proposed coal-fired unit coupled with solar energy and biomass supercritical water gasification not only improves the flexibility of coal-fired power plants, but also offers the potential for reducing coal consumption, efficiently utilizing biomass conversion, and ensuring grid security and flexible dispatch.