Abstract: Eddy current in the containment of electromagnetic railgun is induced by the pulse current, and it will badly weaken the propulsion of the armature and the efficiency of the electromagnetic railgun (EMG) system. In order to investigate the influence of the containment on the efficiency, a full circuit model in combination with a finite element method was employed. Firstly, the launch efficiency and the eddy-current loss were calculated, and the efficiency of different containment structure and materials were discussed. Results show that, based on a medium caliber electromagnetic railgun with 10 MJ pulse power, its stainless steel containment will reduce the efficiency very seriously, and the eddy-current losses in containment are more than half of muzzle kinetic energy. While laminated structure and high permeability containment can improve the efficiency obviously. Lastly, the influence of the containment on mechanical property of the electromagnetic railgun was analysed. As a result, the decrease of the armature axial force by the containment is the primary cause of efficiency decline, and the decrease of armature radial force can increase the contact resistance between armature and rails, which will reduce the efficiency too; in addition, the decrease of the radial force of barrel components helps to reduce the preload.