Compulsators are popular choices for high-end railgun power supplies. In order to maximize energy stores and power densities, compulsators are designed as an air-core prototype. However, new problems have also been brought out by the new structure. Due to the poor thermal conductivity of composite materials and the high field current needed to maintain the magnetic potential, one of the principal limiting factors for achieving continuous discharges is the high winding temperature rise. In this paper, on the basis of electromagnetic analysis, the losses of the coils and compensating shield could be calculated first, and then, a transient 3-D finite-element thermal analysis was performed for both the stator and rotor. Finally, for the purpose of stabilizing the hottest spot temperature of the coils at a given level, an active cooling system has been designed and simulated. The presented method can be applied to other compulsators having the same thermal issues.