Abstract: Long-term high flow rate drainage of precipitation in open-pit coal mines has led to serious waste of groundwater resources in the mine area, rapid degradation of the ecological environment around the mine field, and significant increase in production and operation costs. The curtain interception and emission reduction technology provides a new water control idea for the water control in open-pit mines. The stability of the trench section is directly related to the safety and efficiency of the water cutoff curtain during construction, at the same time, it also has a great influence on the anti-seepage effect. Taking an open-pit coal mine in the eastern Inner Mongolia of China as an example, we used the two-dimensional and three-dimensional horizontal slice method to study the factors and laws affecting the stability of the long trench section, in order to explore the stability of the trench section in sandy cobble stratum during construction. The differences between the two-dimensional and three-dimensional horizontal slice methods in the calculation of safety coefficient were compared and analyzed. The stability of the trench section was simulated and analyzed with the help of FLAC^3D software for two lengths of 17.5 and 22.5 m for the sand cobble stratum respectively. Combined with the above analysis results, the stability control measures of the long trench section of sand cobble stratum were derived and verified by ultrasonic testing. The results of the study show that the main factors affecting the stability of the trench section are: the nature of the formation geotechnical body, the excavation depth of the trench section, the mud level, the mud density, the difference between the mud level and the groundwater level and the length of the single width trench section. The stability of the long trench section can be increased by adjusting the mud parameters, increasing the mud level, lowering the groundwater level, optimizing the trenching sequence and controlling the trenching time. If the length of a single trench section does not exceed 21 m, the trench can be kept upright and stable, and the long trench technique is feasible.