A high-precision motion control system for an in-plane longitudinal bending composite mode linear piezoelectric actuator is proposed. This system can realize precise output control with nanometer position error and millimeter velocity error and ensure accurate tracking of the predefined reference trajectory. The control system combines a sliding-mode state observer（SMO）with a disturbance observer（DOB）for real-time observation of the system state and external disturbances. The position sensor and SMO are used to estimate the system state values accurately，and the radial basis function neural network controller is used to adjust the parameters adaptively online to ensure precise control of the piezoelectric actuator based on the error-generating control law. The synergistic effect of the DOB and SMO significantly reduces the observation error and strengthens the resistance of the system to disturbances. In addition，this study derives the stability of the control law based on the Lyapunov stability，and the effectiveness of the method is verified through simulations and experiments. The actual position error is controlled within ±60 nm，and the velocity error is controlled within ±0.025 mm/s.