Abstract: At present, because of reaction torque, impact load and other external force, the screw motor shell tripping accidents often occurred in the process of underground horizontal directional drilling. According to the screw motor shell tripping accident of underground horizontal directional drilling test in Songzao Coal Mine, some aspects were analyzed, such as structure, indoor test, geological conditions and so on. The development of high torque and high strength screw motor, the technical improvement measures of connecting thread were proposed, which could ensure that the screw motor worked stably and prevented the connection thread tripping of screw motor shell.