Based on electrolytic principle and the kinematics analysis for single abrasive grit with ultrasonic vibration, the mathematical model of grinding force for ultrasonic ELID composite grinding is proposed. The analysis and simulation are conducted to the model. The analysis of the model shows that:ultrasonic vibration changes the motion trajectory of the abrasive grains, and the undeformed chip thickness is reduced under the same conditions. The inprocess dressing of grinding wheel maintains the abrasive grits in the fengrui state, but also affects the actual depth of cut of the grinding wheel. All that will impact on the grinding force. The grinding force decreases with the increase of ultrasonic vibration frequency and amplitude, the electrolysis voltage, the pulse ratio, the electrolyte resistivity, and increases as the depth of cut, the workpiece speed increases.