Mathematic model of heat transfer in round crystallizer with water cooled jacket in horizontal continuous casting for iron bar was established. Heat transfer process of water cooled jacket system with various height width ratio of the water channel was calculated by the finite differential method. With increasing value of height width ratio of water channel K

both the velocity of water flow in the cooled jacket and equivalent heat transfer area of water channel section decreased so that the cooling capacity of cooled jacket was weaken. The design principle for water cooled jacket in round cystallizer was provided based on the calculated results.