On the basis of the particle scattering imaging, the refractive index of the particle can be obtained by the distance between the zero-order and the first-order spectra on the focal plane. The influences of the received angle and the magnification deviation of the imaging system on the measurement accuracy of the particle refractive index are analyzed. And the ranges of the received scattering angles and the magnification are given. An experimental method based on linear fitting is proposed to accurately determine the scattering angle and the magnification. The refractive indices of standard polystyrene (PSL) particle with diameter of 45 μm and glass particle with diameter of 19.1 μm are measured with the proposed method, their relative measurement errors are -0.19% and 1.53%, respectively. The experimental results verify the effectiveness of the proposed method.