This paper aims to study the saturation characteristics for stability of hydro-turbine governing system with surge tank. Firstly, the mathematical model of hydro-turbine governing system under load disturbance is established, and the equivalent independent stability discriminants are presented. Then, the saturation characteristics for stability are analyzed. Using analytical stability discriminants and stable domain, the generation mechanism of saturation characteristics is revealed, and a distinguishing method of critical saturation state is proposed. Finally, the concept of saturation sectional area of surge tank is proposed. The distribution and partition for stability states is illustrated, and a combined tuning method of sectional area of surge tank and governor parameter is proposed. The results indicate that, for hydro-turbine governing system with surge tank, the fourth-order discriminant boundary is always the critical boundary of stable domain. The second-order and third-order discriminant boundaries determine the stable domain under saturation state. The equality for fourth-order discriminant has two real solutions when the system is saturated. The saturation sectional area of surge tank is the sectional area that makes the system reach critical saturation state. Domain C of the distribution and partition figure for stability states is the most favorable domain for the system stability. (C) 2018 Elsevier Ltd. All rights reserved.