According to the principle of grain refining and slurry preparation by cooling sloping plate process, the distributions of boundary layers during melt treatment by cooling sloping plate were studied, and mathematic model of cooling rate was established. The calculation value approximately agrees with the experimental result. Laminar flow and turbulent flow exist on sloping plate surface commonly. The thickness of velocity boundary layer and the critical transfer distance from laminar flow to turbulent flow increase with the decrease of initial flow velocity. The thickness of temperature boundary layer increases with the increment of flow distance and the decrease of initial flow velocity. The melt cooling rate and melt thickness have an inverse proportion relationship. The melt cooling rate increases along the plate direction gradually when the initial flow velocity is lower than 1 m/s, the melt cooling rate keeps nearly a constant when the initial flow velocity is 1 m/s, when the initial flow velocity is higher than 1 m/s, the melt cooling rate decreases gradually. The melt cooling rate of cooling sloping plate process can reach 102-103 K/s and belongs to meta-rapid solidification scope. According to the principle of grain refining and slurry preparation by cooling sloping plate process, the distributions of boundary layers during melt treatment by cooling sloping plate were studied, and mathematic model of cooling rate was established. The calculation value approximately agrees with the experimental result. Laminar flow and turbulent flow exist on sloping plate surface commonly. The thickness of velocity boundary layer and the critical transfer distance from laminar flow to turbulent flow increase with the decrease of initial flow velocity. The thickness of temperature boundary layer increases with the increment of flow distance and the decrease of initial flow velocity. The melt cooling rate and melt thickness have an inverse proportion relationship. The melt cooling rate and melt thickness have an inverse proportion relationship. nearly a constant when the initial flow velocity is 1 m / s, when the init The melt cooling rate of cooling sloping plate process can reach 102-103 K / s and belongs to meta-rapid solidification scope.