A method based on methane solubility was proposed to determine the gas pressure of a coal seam after a borehole was invaded by formation water. First, methane transport equations in a water-saturated coal seam and a water-filled borehole were established, which considered the effect of stress, density-driven convection and solute diffusion. Numerical simulation software was used to investigate the characteristics of the methane dissolution and transport. Second, we established a set of measuring devices to test the methane concentration in the borehole water. Using the No. 10 coal seam of the Taoyuan Coal Mine as the focus area, we tested the dissolved methane concentration over time, and the results were well correlated with the numerical simulation. The coal seam gas pressure was calculated to be 0.35 MPa by testing Henry's constant of the field water. Finally, the applicability of this method was analyzed. The results showed that the effects of stress and convection had little impact on methane transport, indicating that this method was less affected by external influences. This paper provides a new idea for gas pressure measurement in water-rich coal strata.