Abstract: When using open core tube sampling to measure the gas content in deep hole coal seams, the coal sample has a long exposure time. It is prone to pollution, and the gas loss is difficult to measure, resulting in large measurement errors. However, most of the existing sealed pressure maintaining sampling requires secondary expansion and complex structure. The sampling success rate is low, making it difficult to promote and apply on a large scale. In order to solve the above problems, a sealed pressure maintaining sampling device for deep holes in coal mines has been developed. The device adopts an overall structural design of "two-stage piston+double-layer casing+coal sample cylinder". By controlling the injection flow rate and injection time of the underground high-pressure water pump into the device, the ball valve switches from being sealed before sampling to being opened during sampling, and then to being sealed after sampling. The goal of sealing the coal sample in the coal sample cylinder is achieved, preventing contamination of the coal sample and gas escape during the sampling process. Four sets of coal samples from the same borehole and depth are collected using the conventional sampling method of an open core tube and a deep hole sealed pressure maintaining sampling device for comparative analysis of gas content. The on-site test results show that the sampling depth of the deep hole sealed pressure maintaining sampling device is close to 350 meters. Compared to conventional sampling methods, the gas content measured by deep hole sealed pressure maintaining sampling is 5.4% to 37%. The degree of deviation increases with the increase of sampling drilling depth. It indicates that the sealed pressure maintaining sampling device can improve the accuracy of deep hole coal seam gas content testing.