An adaptive backstepping sliding mode control algorithm based on the nonlinear extended state observer (ESO) is proposed for an oscillation displacement system of continuous casting mold driven by servo motor, in which there exists unmatched perturbations including parameter uncertainties and disturbances. Firstly, based on the hyperbolic tangent function and a high gain, two nonlinear ESOs are constructed to estimate the unmatched perturbations. Then, by means of the backstepping technique, the unmatched perturbations can be viewed as the matched ones to be handled, and the adaptive integral sliding mode controller is designed in each step of the backstepping procedure to eliminate the effect of the unmatched perturbations. In addition, two integral sliding mode filters are introduced to estimate the time derivatives of the previous step virtual controllers in the last two steps. It can be proved that the estimation errors of the ESO are convergent to a small neighborhood of the origin, and all of the signals of the closed-loop system are bounded, and the system states can be asymptotically stable under the conditions of selecting appropriate parameters. Finally, simulation results show the effectiveness of the proposed method.