The invention discloses a self-balancing siphon drainage method by using a side slope declining drill hole. A siphon drainage pipe is introduced to the deep part of a side slope through a downward declining hole, a relative height difference between the opening and the bottom of the declining hole is kept less than 9m, and the siphon drainage pipe is introduced to a certain position of a slope surface from the bottom of the declining hole so that a water inlet and a water outlet of the siphon drainage pipe have the same elevation, and water is drained in real time when the water level of a slope body is raised. By adopting the self-balancing siphon drainage method provided by the invention, the condition of draining water at the deep part of the side slope can be realized, the problem of drainage treatment of big landslides can be solved, the low construction cost for maintaining side slope stabilization can be ensured, the earth and stone excavation is unneeded in the treatment process, the implementation process of treatment engineering is simple and easy to operate, drainage measures are favorable in reliability and strong in adaptability, and the power and the frequent management maintenance are unnecessary in the drainage process.