The commonly used Power Back-Off method can improve the nonlinear problem of the amplifier to some extent, but its application scope is limited due to its low efficiency. Based on the Power Back-Off, this paper proposes a method combining polynomial fitting to improve the 6~10 dB backoff range constraint of the traditional Power BackOff method. In order to verify the effect of the combined method, the photoelectric amplification test platform based on LabVIEW was built in this paper to carry out signal amplification tests on sine wave, square wave and triangle wave signals at different given frequencies. When the amplifying function of the system is tested, the output signal is analyzed by power cycle map, and the harmonic distortion state can be determined. The experimental results show that the improved method can reduce the power value of each harmonic component by 34.6% compared with the Power Back-Off, which proves the effectiveness of the proposed method to further improve the nonlinear problem of the amplifier.