Limestone with a particle size of less than 5 mm was rapidly calcined in a high-temperature resistance furnace at 1623 K to simulate the conditions of rapid calcination of limestone at ultra-high temperature in a converter. In this study, the decomposition mechanism and calcination characteristics of small-sized limestone at steelmaking temperature were investigated. The study shows that the shrinking sphere or cylinder models with phase boundary reaction were found to be the best representation of limestone kinetic data, and the mechanism function equation is G(α) = 1 − (1 − α)n, n = 1/2 or 1/3. Limestone particles with a size of 0.18−1.0 mm can be quickly calcined to obtain a typical active lime microstructure and a high activity of more than 350 mL, which is the preferred limestone particle size range in the steelmaking process in which limestone powder is injected into the converter.