Abstract: According to the characteristics of aluminum cast-rolling process, the authors built the basic equations of roller-board system's temperature fields. The authors clarify in detail that the cooling effect on cast-roller's surface is better than that on board surface, so the former is our main direction. The authors also reveal the vary rule of the best external-cooling area: The best external-cooling area of super-thin high-speed cast-rolling is twice more than that of normal cast-rolling, further more, after the roller's surface has been cooled, the best external-cooling area will be shortened and moved to the exit side of casting area. According to the law, it will make the cooling ability of external-cooling equipment to regulate the interrelated parameters of external-cooling spray gun.