Abstract: Aiming at the inherent limitations of directional drilling flushing fluid open circulation in borehole measurement while drilling, a scheme of closed circulation technology for washing liquid purification by mechanical solid-liquid separation is proposed. Based on the environmental conditions of underground coal mine construction, the characteristics of solid phase and physical properties of flushing fluid and the requirements of directional drilling cycle, the technology of closed circulation of flushing fluid in underground coal mine was developed, and the two-stage mechanical flushing fluid closed circulation device for mine with rated treatment capacity of 200 L/min was developed, and the corresponding technological process was designed. The field test results show that the two-stage mechanical closed-cycle device for flushing fluid can effectively separate the multiphase fluid of gas-liquid-solid returned from the borehole underground coal mine. The solid content of the flushing fluid after treatment is less than 0.15%, and the particle middle-size of the solid content is less than 10 μm, which meets the normal working requirement of the screw motor at the bottom of the borehole. The mechanical closed-cycle technology and device is of great significance for innovating the circulating mode of flushing fluid in directional drilling and reducing the amount of sewage.