The contour intersection method is a novel method for inverting the complex refractive index of small particles. Currently, there are no reports on the use of this method for the inversion of the complex refractive index of nanoparticles. In this study, Au nanospheres are considered as the research object, and the feasibility and reliability of the contour intersection method in the inversion of the complex refractive index of nanoparticles are investigated. The Mie theory and dielectric function size correction model are used to calculate the relationship between the light scattering and absorption characteristics of Au nanospheres and the complex refractive index. The complex refractive index of the particles is obtained by a combination of the contour intersection method, Mie theory, and dielectric function size correction model. The backscattering efficiency constraint method is proposed for determining the unique solution among multiple valid solutions for the contour intersection method. The effects of the complex refractive index step size, particle size, and measurement error on the inversion results are quantitatively analyzed. Finally, the inversion accuracy of this method is compared with that of the traditional iterative method. The results show that if the scattering light efficiency and light absorption efficiency of Au nanospheres are known, the accurate complex refractive index can be obtained using the contour intersection method; when the measurement error is less than 5%, the accuracy of the inversion results can be ensured; under the same conditions, the inversion results of the contour intersection method are better than those of the iterative method. This simple inversion method is reliable for the measurement of the complex refractive index of Au nanospheres.