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Stable adhesion on non-magnetic, steep, and irregular underwater surfaces (e.g., concrete dams with cracks or biofilms) remains a challenge for inspection robots. This study proposes a Venturi-ejector composite suction cup synergizing a rigid base with a dual-layer EPDM sponge seal (closed-cell + open-cell) for adaptive conformability. This design enables high-efficiency negative-pressure generation through hydrodynamic entrainment, overcoming limitations of traditional adsorption methods. Theoretical modeling established the quantitative relationship between adsorption force (F) and key parameters (nozzle/throat diameters, suction cup radius). CFD simulations revealed optimal adsorption at a nozzle diameter of 4.4 mm and throat diameter of 5.8 mm, achieving a peak simulated F of 520 N. Experiments demonstrated a maximum F of 403.1 N at 88.9 W power. The composite seal significantly reduced leakage on high-roughness surfaces (Ra≥6 mm) compared to single-layer designs. Integrated into an inspection robot, the system provided stable adhesion (>600 N per single adsorption device) on vertical walls and reliable operation under real-world conditions at Balnetan Dam, enabling mechanical-arm-assisted maintenance.