Beginning with the reasons of chattering, proposes an improved sliding mode reaching law, which adds an exponential term on the bi-power reaching law and gives a specific method for calculating the arrival time of the control system to the sliding surface. It overcomes the defects of the single power reaching law which approaching the sliding surface for a long time and chattering bigger, and ensures dynamic quality of the system. Applies it to a two-order system, and makes a comparison with the single power reaching law, the simulation results verify the effectiveness of the proposed approach.