Estimation of the secondary path model is required in the implementation of the Fx-Newton algorithm. Aiming at the problem that errors exist in the estimation of the secondary path model in the active-passive vibration isolation engineering application, providing that the input signal is a sine signal, the structure model of the Fx-Newton algorithm was established, considering the error of the estimation of the secondary path model. Stability condition of the Fx-Newton algorithm was deduced. Effects of the amplitude and phase errors on the stability and convergence of the Fx-Newton algorithm were elaborated. Finally, simulations for a two-degree-of-freedom active-passive vibration isolation model were conducted and the theoretical analysis results were verified.