Abstract: Drainage is an effective method for landslide control. Siphon with free power can realize water transport and the flow process controlled by liquid level change. It is very suitable for the need of slope groundwater control. But the highest siphon head 10m and intermittent drainage may produce siphon top air accumulation, which restricts the application of landslide drainage. Using declined drilling into the deep slope, and keeping the difference between the groundwater level control point height and the orifice height less than 10m, can realize the landslide deep groundwater descending requirements. Controlling the siphon drainage pipe diameter, the siphon tube to form a stable slug flow can prevent air accumulation. So, it can realize the continuous siphon process. Based on a thin seam model, it has proved that the maximum diameter of the siphon to form stable slug flow is about 4mm by energy method derived. The long-term static model also verified that 4mm diameter PA tube can form a complete slug flow. Examples are given to demonstrate the drainage project selection 4mm siphon, which can realize the landslide deep drainage and automatic restoration of siphon in intermittent process.