To address the issue of high-velocity flow easily inducing strong hydraulic oscillations in the "Y-shaped" diversion outlet layout at the end of steep slopes, this study analyzed the causes of water surface fluctuations and the core mechanism of hydraulic oscillation formation using engineering case studies combined with model tests. It proposed wave-dissipation engineering measures and evaluated their effectiveness. Dual verification through both model tests and engineering practice demonstrates that when high-energy flow enters the enclosed boundary energy dissipation structure, a standing wave system forms on the outlet side, causing periodic violent water surface fluctuations and hydraulic oscillations. After implementing wave-dissipation measures: Under the wave-dissipation baffle scheme, wave reduction amplitudes reached approximately 70% at low flow, 60% at medium flow, and 78%~84% at high flow. Under the deflection baffle scheme, wave reduction amplitudes reached approximately 70% at low flow, 55%~75% at medium flow, and 70%~80% at high flow. Experimental studies confirm that both wave-dissipation baffles and deflection baffles achieve significant wave attenuation effects. They effectively resolve hydraulic oscillation issues and eliminate the severe intermittent "slamming and flipping" phenomenon (commonly known as "pāimén and fānzhá") at the outlet gates. These findings provide crucial technical support for the design and operation of similar water conveyance structures.