This paper presents the results of a computational fluid dynamic model of a supersonic through-flow fan (STF) in supersonic surge. The phenomenon of surge is well known for subsonic turbomachinery. However, for supersonic turbomachinery like the STF, a special type of supersonic surge occurs in which a shock oscillates through the fan, alternating locations between upstream and downstream of the fan. It is also possible for a shock to remain in a fan stage. This analysis focuses on the development of supersonic surge resulting from overpressuring the fan. A model of the STF was constructed around its steady-state experimental performance map using an existing unsteady, quasi-one-dimensional, inviscid, compressible flow code. An exit boundary condition was specified for the overpressuring that forces the shock to move upstream and interfere with normal fan operation. The effects of that interference, including a shock oscillating across the fan, are discussed.