Abstract: In underground coal mine long borehole sampling, significant challenges include substantial gas loss, complex borehole conditions, and low coal sample drilling rates. To address these issues, a pressure-preserving sampler is designed, integrating high-pressure water as the driving force, a piston drive, and a ball valve seal. This sampler controls the opening and closing of the ball valve through water pressure changes transmitted via the drill rod, enabling efficient coal sample drilling and pressure-preserving sealing. Experiments using the sampler included three sets of conventional borehole samples and eighteen sets of long borehole samples, with a maximum sampling depth of 297 meters. The sampler ensured coal seam exposure time during sampling was controlled within 5 minutes. Seam gas content measurement results indicated that, within the same working face, the gas content measured by the pressure-preserving sampler was 0.01-1.34 m³/t higher than that from conventional methods. Additionally, the sampling depth of long boreholes showed no significant impact on gas content values, demonstrating the sampler's effective sealing and pressure-preserving performance post-sampling. The pressure-preserving sampler successfully fulfills the objective of measuring coal seam gas content in underground coal mine long borehole sampling.