To study the hydraulic characteristics and applicability of the free-surface-pressure flow tailwater system in the water conveyance and hydropower system, an analysis model for the transition process of the water conveyance and hydropower system was established based on the method of characteristics and the characteristic implicit format. The hydraulic characteristics of two types of tailwater systems, namely, the free-surface-pressure flow and the full-pressure flow, were compared and analyzed. Furthermore, the influence of the transition process of the free-surface-pressure flow tailwater system on the transient parameters of the water conveyance and hydropower system and the regulating quality of the units was further investigated. The results show that, under the conditions of full-pressure flow and free-surface-pressure flow in the tailwater tunnel, the regulating quality of the units ensures that the controlled value deviation of the calculated parameters is small, and the minimum internal water pressure at the inlet of the tailwater pipe corresponding to the free-surface-pressure flow tailwater system is relatively large. The regulating quality of the units of the free-surface-pressure flow tailwater system is better than that of the full-pressure flow tailwater system, and there is no obvious high-frequency oscillation superposition phenomenon.