This paper presents a study of flow characteristics in high-lift siphon drains. A laboratory test was conducted to investigate the effects of hose diameter and flow velocity on siphon drainage. Three types of water flow were observed and analyzed. The experimental results show that the flow characteristics of siphon drainage are significantly influenced by the hose diameter. Water flows in the form of a wall pressing flow in a large diameter siphon hose (e.g., larger than 5.0 mm) under low flow velocity, which leads to discontinuous siphon drainage. However, water flows in the form of an integral slug flow in a small diameter siphon hose (e.g., smaller than 4.0 mm) under low flow velocity, which leads to continuous siphon drainage. Based on experimental observations, a thermodynamic derivation of the threshold of siphon hose diameter for continuous siphon drainage was analytically conducted. In slope engineering, a 3.6 mm polyurethane (PU) hose is recommended for siphon drainage.