To investigate the erosion-corrosion behavior of nitrogen-carbon-oxygen complex treatment (QPQ) layers and high-velocity oxy-fuel (HVOF) coatings on 17-4PH stainless steel in a liquid-solid two-phase flow environment. The effects of flow velocity, impact angle, and fluid medium composition on the erosion rate of the material were investigated through liquid-solid two-phase erosion tests and salt-containing erosion-corrosion experiments. The results show that, the erosion rate increases with increasing flow velocity within 90 min, and the coatings remain structurally stable during the test period. At the same flow velocity, the erosion rates of the HVOF coating and the QPQ layer are approximately 4% and 13% of that of the substrate, respectively. The most severe damage of the QPQ layer occurs at an impingement angle of 45°, and local spallation appears after 60 min. For the HVOF coating, the maximum erosion rate is observed at 90°, while the minimum value occurs at 30°. In a mass fraction of 3% NaCl medium, the erosion rates of the substrate, the QPQ layer, and the HVOF coating increase to 1.97, 2.78, and 1.83 times those in fresh water, respectively, indicating a pronounced synergistic effect between corrosion and particle impact. It can therefore be concluded that the medium environment plays a decisive role in the erosion behavior of materials. Among the studied surface treatments, the HVOF coating exhibits the best resistance to erosion-corrosion, whereas the protective capability of the QPQ-treated layer decreases markedly in chloride-containing environments.