Stickslip and friction of the joint interface pay a significant role not only on structure nonlinear stiffness and damping but also on structure passive damping. Iwan model provides a preferable recurrence of the stickslip and friction characteristics of the joint interface. The alternating frequency/time domain method (AFT) is used to study the steadystate response of single degree of freedom oscillator system containing Iwan nonlinear model in this paper. This method takes both advantages of the effectiveness of calculating response for linear system by frequency domain method and the ease of evaluating nonlinear force by time domain method. The discrete Fourier transform alternating from the time domain to the frequency domain is applied through iterating to obtain the steadystate response of the system. The result of AFT is compared with that from the central difference method, and the influence of excitation amplitude on nonlinear characteristic is also examined in this paper. The results show that the steadystate response by AFT method agrees well with that of the central difference method. Moreover, the AFT method performs better computational efficiency, as the timeconsuming is reduced by 50 percent. In a whole, it is found that the application of higher excitation amplitude results in the increase of the system energy dissipation and the reduces stiffness and natural frequency.