Based on the rigid water column theory and the pipe system of a practical hydropower station with bifurcation downstream tailrace surge tank, a set of state equations to describe stability of the gover-nor-turbine-hydraulic system under small perturbation is developed. The effect of throttle orifice and charac-teristics of turbine and governor are considered in the model. The stability of the system is analyzed with the application of the state equations and stability theory, which is verified by numerical model of tran-sients based on the Method of Characteristic (MOC). The influence of sectional area of surge tank and throttle orifice on the stability is also investigated. The results show that the system becomes more stable with the increase of surge tank sectional area and the decrease of throttle orifice sectional area. When the area of surge tank is larger than the stable sectional area, system’s stability is not sensitive for the in-crease of the surge tank area. However, the stable area of the system decreases obviously as the surge tank area becomes smaller, when the area of surge tank is less than the stable sectional area. The size of throttle orifice can influence the stable sectional area, and the stable sectional area decreases when the ar-ea of throttle orifice become smaller.%基于刚性水锤理论，结合实际工程水道布置，考虑调压室阻抗作用以及实际水轮机和调速器特性，对采用尾水岔管室后交汇布置型式的水电站推导了水力机械系统小波动稳定性分析状态方程。运用稳定性理论，分析了电站的小波动稳定性，并利用基于特征线法的水电站过渡过程数值模拟进行验证，两者结果基本一致。随后研究了该类型调压室面积及阻抗孔口对系统稳定域的影响。结果表明：调压室面积越大，阻抗孔口面积越小，系统越稳定；当调压室设计面积超过稳定断面面积时，进一步增加调压室横截面积，对系统的稳定性并无多大改善，而当面积小于稳定断面面积时，系统的稳定域大幅度减小；阻抗孔口的尺寸会影响调压室的稳定断面面积，阻抗孔口尺寸越小，对应调压室所需的稳定断面面积越小。
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