Baihetan Hydropower Station has a huge capacity of single unit with the layout of two units sharing a common tailrace surge chamber and tailrace tunnel.Since various and complex flow pattern sexist in the tailrace tunnel, analysis and control of hydraulic disturb transition is an important prerequisite to ensure safe and stable operation of the system. Based on the Method of Characteristic for pressurized water conveyance system, the characteristic implicit format of free-surface flow analysis was introduced, and the mathematical model of the free-surface-pressurized flow in tailrace tunnel was derived.The stability analysis and regulation model of water conservancy and hydropower system was constructed considering the frequency regulation mode of the governor and stability analysis of hydraulic disturb was analyzed. The results show that the transition process of hydraulic disturb in Baihetan Hydropower Station is stable with the effect of various flow patterns in tail tunnel system. With the increase of unit operating head, the tailwater system transits from completely pressurized state to free-surface-pressurized flow state and free-surface flow state.The length of pressurized part decreases, and the water level of the tailwater surge chamber and the dynamic process of tailwater system decay faster, which is conducive to improve the regulation performance of the disturbed units. Both the two different unit types meet the stability requirements on hydraulic disturb of water conveyance and hydropower system, and the regulation performance of the disturbed units is basically the same. Keywords: water conservancy and hydropower system; hydraulic disturb; stability analysis; free-surface-pressurized flow;Baihetan Hydropower Station 〖FL