Unreasonable spacing of long boreholes in this coal seam causes ineffective borehole overlap and gas-drainage blank zones. A representative case of the 1692(1) working face in Pansan Mine was considered. A gas-solid coupled model was established on the COMSOL Multiphysics platform. The gas-pressure distribution around long boreholes under different drainage times and its influence on the effective drainage radius were simulated and analyzed. The simulation results show that gas pressure along the borehole decreases non-uniformly. A stronger drainage capacity is observed in the front and middle sections of the borehole. Therefore, a marked pressure drop is formed around these sections. In the rear section, especially within 10 m from the borehole bottom, the pressure drop is limited. As a result, the effective drainage radius is significantly reduced. Gas-pressure profiles at different borehole positions are extracted. Gas-pressure variations on each profile under different drainage times are analyzed. Characteristic curves of the effective drainage radius versus time are obtained. The fitting results indicate that the effective drainage radius has a significant exponential relationship with drainage time. The effective drainage radius increases with drainage time. However, the increase differs by position. The radius near the borehole bottom remains smaller than that at the borehole mouth and the middle section. If borehole spacing is designed by the radius at the borehole mouth, a drainage blank zone is formed. If borehole spacing is designed by the radius at the borehole bottom, redundant borehole overlap is produced. Therefore, the effective drainage radius at 10 m from the borehole bottom is recommended as the basis for borehole-spacing design. The last 10 m of the borehole is defined as a pressure-overlap section. In this way, blank zones are avoided, and both drainage performance and engineering economy are ensured. These results provide theoretical support and numerical evidence for rational borehole-spacing design for long boreholes in this coal seam.