As power supplies, compulsators are popular choices for electromagnetic railguns. Compare to single phase system, multiphase compulsator can provide more flexibility in current waveform and higher power density. However, the accurate simulation method of a multiphase compulsator is still not clear enough for researchers since both the machine and the external circuit become more complex. In the view of this background, this paper describes a finite-element method combined with circuit analysis. The machine is modeled in Maxwell while the railgun launcher and external circuits are modeled in Simplorer taking their respective advantages. Then co-simulation model is constructed by simulation platform ANSYS using its powerful co-simulation features. Based on this model, the performance of the two and three phase air-core compulsator powered railgun systems are analyzed. The compensation modes, trigger control and output characteristics are also discussed. The co-simulation model and the results can provide the theoretical guides for multiphase compulsator investigation in the future.