Abstract: When measuring pressure in coal seam gas, water accumulation in downward boreholes will lead to inaccurate pressure measurement.It is an urgent problem to be solved to develop a device that drainage without affecting pressure measurement in and its result downward drilling pressure measurement. Therefore, the automatic drainage module was designed, and the elastic membrane was used to sense the drainage time in real time to realize the automatic drainage. The elastic parameters of the elastic membrane were studied experimentally, and the membrane thickness of 0.3 mm to 1.0 mm was selected. Based on the principle of gas pressure difference and capillary force, the optimal drainage interval was calculated. On this basis, an automatic drainage experimental device for downward drilling of coal seam gas pressure measurement was developed and the drainage test was carried out. The test results show that when the pressure of pressure chamber is 0.152 × 10⁵ Pa, the diameter of drainage pipe is 15 mm, the inner diameter of capillary is 0.2 mm, and the number of capillary is 950, the drainage speed is 0.444 3 cm³/s, the device can achieve continuous slow drainage.