Abstract: The thermal diffusion characteristics of mushroom head type outfalls in unidirectional water areas are influenced by various factors, including discharge jet, environmental flow, and density flow. Understanding the hydraulic and thermal behavior of these outfalls is crucial for guiding engineering design. In this study, a three-dimensional numerical simulation method was employed to investigate the hydraulic and thermal characteristics near a mushroom head type outfall (four-sided window) in a unidirectional flow environment. The results indicate that as the unidirectional flow velocity increases, the warm drainage from each window deviates further downstream. The reverse flow in the opposite window decreases with higher unidirectional flow velocity, while the flow in the direction window remains less sensitive to the unidirectional flow. Additionally, the direction of the window jet transitions from an upward flow on the surface to a downward flow toward the seabed as the unidirectional flow velocity increases. Moreover, as the velocity ratio increases, the temperature rise distribution in each window initially converges and then gradually assumes a “Yuanbao” shape. Furthermore, an increase in unidirectional flow velocity effectively reduces the volume of water with a temperature rise of 1.0 ℃.