A full transient elastohydrodynamic lubrication(EHL) solution of involute spur gears is obtained under impact load based on the multi-grid (MG) method for solving the pressures, the multi-level multi-integration (MLMI) approach for evaluating the elastic deformations, which takes into account the variation of equivalent curvature , entertainment velocity and load on time along the line of action, the gear tooth surface is assumed to be smooth. The influences of impact load spectrum and the approach impact load when the teeth come into action on the EHL pressure and film thickness are analyzed in the paper. The results show that the approach impact load can strongly influence the approach point maximum pressure and minimum film thickness. The impact load can lead to instantaneous lubrication film deterioration between contact teeth of involute spur gears. The maximum pressure and the minimum film thickness all occur in the vicinity of approach point immediately after the impact load was feed. The approach impact load is seriously harmful to the gear lubrication.