In order to meet the high performance control requirement of manipulator arm system, the exponential reaching law was analyzed and manipulator arm's dynamics characteristics were investigated. On the basis of it the exponential reaching law was improved and its existence was proved. According to the improved reaching law, the sliding mode control strategy was designed and simulated through Matlab with a manipulator arm as the control object. The simulated results show that the method is able to meet the system fast tracking and restrain the chattering problem in the sliding mode variable structure control.