The steady temperature field of the new cooling stave of cast steel is numerically simulated. The result shows that the maximum temperature of its hot-side, within the service life that fins disappear, is still in the safe range even if the slag on hot side breaks off. And the temperature is merely 28℃ higher than copper cooling stave's when slag of 10 mm thickness is attached to each of them. The temperature difference between them becomes less when the thickness of slag gets thicker, and eventually tiny.