the two-degree-freedom dynamics equations that describe the non-linear vibrations of non-symmetrical cross-section are established condition in large rotation. The equations are simplified by ignoring square non-linear terms and retaining linear coupling and cubic non-linear terms. The simplified equations are solved by the multiscale method for harmonic vertical load and harmonic torque. The results illustrate that the cubic non-linearities play a role of hard spring when the external torque induce the resonance of low-frequency, and the cubic non-linearities play a role of soft spring when the vertical load induced the resonance of high-frequency. And as a result of the cubic non-linearities the vibration amplitudes may be suddenly jump with the change in excitation amplitude and excitation frequencies. This does not appear in the linear differential equations that is only considered a small torsion.