An approach to reduce the effect from source nonlinearity and improve the measurement accuracy is proposed when determine the nonlinear parameter of material using finite amplitude method. Based on quasilinear theory, the fundamental wave, second harmonic wave and source nonlinear wave sound fields are described as the combination of pure plane solutions, diffraction correction terms and attenuation correction terms using multi-Gaussian beam model. The amplitude of source nonlinearity is determined through the output monitoring signal of the equipment. The expression of determining nonlinear parameter based on plane wave is corrected, in order to reduce the effects from sound diffraction, attenuation and source nonlinearity. The nonlinear parameter of Al-6061 specimen is measured experimentally, and the results show that the errors of measured nonlinear parameter taking consideration of source nonlinearity in different input voltages are controlled in 10%. The precise has been of great improved compared with that without considering the source nonlinearity. This research will provide a theoretical method to reduce the effect from source nonlinear and improve the precise of nonlinear parameter of material in experiment.