Considering the electrorheological(ER) damper applied in the gun recoil device under the high impact load, the structure of damper adopts the fixed electrode. The twodimension (2D) symmetrical geometry model of ER damper was built by using COMSOL Multiphysics field software. According to Computational Fluid Dynamics (CFD) module, simulation for the laminar flow of an incompressible ER fluid was derived. The phenomenon was analyzed of fluid flow in the annular gap and the magnitude of damping force in the varying electric field under high speed impact. The structure parameters of ER damper that affected the output of damping force were also researched. The simulation results show that increasing the voltage from 1 kV to 5 kV applied on the electrode, flow velocity decreases by 103 m/s but the total damping force increases by 375.9 N in the annular gap. Under condition of 5 kV/mm of electric field intensity, the damping force increases from 828.2 N to 1 950.8 N with the increased 25 mm effective length of damping channel; and effective length of damping channel stays the same, the damping force decreases to 113.3 N with the increased 1.5 mm annular gap.