Electrorheological finishing method is a promising method for small parts fabrication. The small footprint of the tool can provide material removal by virtue of numerical control machine. The characteristics of electrorheological polishing fluid is tested by Haake CV20 rheometer. The dependence of shear stress and viscosity of the electrorheological polishing fluid under different supply voltage, as well as the field dependence of shear stress of ER polishing fluid at different shear rate, was obtained. Hydrodynamic pressure model in working area was studied based on the phenomenon of hydrodynamic lubrication theory of Bingham fluid, and it is proved that the pressure distribution is the dominant factor on the footprint shape. The experiments further showed that the efficiency of material removal is a combined action of hydrodynamic pressure and wheel speed.