<p>Nonlinear flutter response estimation is important for structural safety and serviceability design. Traditionally, the free vibration test is used to identify the parameters of the fluid-structure interaction, but the corresponding frequencies are very limited. The forced-motion apparatus can measure the aerodynamic force at different frequencies, but can only provide the aerodynamic force and cannot predict the response directly. This study proposes a method to predict the nonlinear flutter response through the aerodynamic force measured by forced-motion equipment. At first, a nonlinear aerodynamic model combines the traditional flutter derivatives for the linear part and the additional polynomials for the nonlinear part, which is identified and fitted by the aerodynamic force autonomous inference method. In the second part, the differential equations of the flutter amplitudes and the phase difference are derived by complexification-averaging methods. The flutter amplitudes at the stable limit-cycle oscillation stage can be calculated when the differential equation equals zero. The amplitude ratio and the phase difference between vertical and torsional motion can also be calculated by the proposed methods.</p>