MHD instabilities during current quench and runaway electron plateau have been observed in experiments, in which runaway electrons play an import role. To simulate these instabilities, a model of runaway electron needs to be included in the simulation and coupled to MHD equations. One simplified way is a fluid model proposed in Helander (2007), which utilizes the relativistic effect of runaway electrons and only simulates the density evolution of runaway electrons instead of the whole distribution in momentum space, to save computation time. In this work, we implement a fluid model of runaway electrons in M3D-C1 code to simulate resistive MHD instabilities happened during disruptions. We first study the linear growth rates of m=1 and m=2 tearing modes, and compare with the results in Helander (2007). It is found that in addition to exponential growth, the MHD instabilities also have a real frequency with a sign depending on runaway electron streaming direction. The perturbed current will have double-layer structure, one of which depends on resistivity and the other depends on vA/c. Then we study the nonlinear evolution of MHD instabilities with runaway electrons, and benchmark with one DIII-D experimental shot.