Particle size is an important factor for determining the concentration of suspended particle matter (SPM) in water using optical remotely sensed data. We collected reflectance spectra of five SPM samples with different particle sizes in a controlled laboratory experiment using a spectroradiometer. The theoretical relationship between particle size distributions and backscattering coefficient was deduced based on a spectral reflectance model. The backscattering coefficient of the complete SPM sample can be computed using the linear weighted combination of four percentages of different subsamples. The spectral similarity scale results indicate the optimal optical bands and boundary conditions for particle size and concentration of SPM remote sensing. The particle size can be evaluated by optical remote sensing to improve the applicability and precision of remote sensing models for SPM concentration inversion.