In this paper, we propose an online secondary-path-modeling ANC system with simultaneous perturbation (SP) method. This system updates the noise control filter by the FXNLMS algorithm, which has fast convergence in the case when the modeling error of the secondary path model is small. If the secondary path changes, the noise control filter is updated by the SP method, which does not require the secondary path model and only utilizes the error signal obtained at the error microphone, and the online secondary path modeling is conducted using the perturbation components in the SP method. When the modeling of the secondary path is finished, the FXNLMS algorithm is applied to update the noise control filter by using the updated secondary path model again. Thus, the proposed online secondary-path-modeling ANC system can compensate the disadvantage of the FXNLMS algorithm for the secondary path changes based on the SP method.