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The hydraulic characteristics of the lock chamber section in a bottom outlet are complex. Particularly, cavitation is a prominent problem when the flow in the tunnel transitions from pressurized to unpressurized conditions. As a countermeasure, sidewall contractions are often installed at the end of the pressurized section. In this paper, with numerical simulations and vacuum tank experiments, we analyzed the design parameters of several prototype projects and studied the flow properties and cavitation characteristics of the transition section. The four main factors that influence cavitation risks are the contraction ratio of the transition section, the top press slope, the length of the transition section, and the shape of the orifice. The study of a real-world project demonstrates that by implementing a severe constriction degree, the top press slope can be slowed down, and the pressurized transition section can be extended, thereby reducing cavitation risks and improving flow patterns. These research results provide a reference for the optimal design of hydraulic structures. HIGHLIGHTS This paper analyses four factors that affect the cavitation of the bottom discharge tunnel.; This paper collects the design parameters of 23 lock chambers and summarizes the value range and design law of these design parameters.; The cavitation and flow pattern of the LW hydropower station are verified through the vacuum tank experiment and numerical simulation.;