Flexible manipulator arm occurs twisted, elastic and shearing deformations in the process of movement，which brings difficulty to its analysis and control. In order to meet the requirements of high performance control of flexible arm, proposes the observer-based sliding mode control method for flexible manipulator, designs an observer to monitor the state variables of flexible manipulator arm system, and applies sliding mode variable structure to design the controller. The simulated result shows that the proposed method does well in observing the variables of the system and the state estimation error approaches to zero, which meets the fast tracking of flexible arm, and has good practical significance.