This paper proposes a laser scattering centrifugal liquid sedimentation (LS-CLS) method for the accurate quantitative analysis of the mass-based size distributions of colloidal silica. The optics comprised a laser diode light source and multi-pixel photon-counting detector for detecting scattered light intensity. The unique optics can only detect the light scattered by a sample through the interception of irradiated light. The developed centrifugal liquid sedimentation (CLS) method comprised a light-emitting diode and silicon photodiode detector for detecting transmittance light attenuation. The CLS apparatus could not accurately measure quantitative volume- or mass-based size distribution of poly-dispersed suspensions, such as colloidal silica, because the detecting signal includes both transmitted and scattered light. The LS-CLS method exhibited improved quantitative performance. Moreover, the LS-CLS system allowed the injection of samples with concentrations higher than that permitted by other particle size distribution measurement systems with particle size classification units using size-exclusion chromatography or centrifugal field-flow fractionation. The proposed LS-CLS method achieved an accurate quantitative analysis of the mass-based size distribution using both centrifugal classification and laser scattering optics. In particular, the system could measure the mass-based size distribution of approximately 20 mg mL poly-dispersed colloidal silica samples, such as in a mixture of the four mono-dispersed colloidal silica, with high resolution and precision, thereby demonstrating high quantitative performance. The measured size distributions were compared with those observed through transmission electron microscopy. The proposed system can be used in practical setups to achieve a reasonable degree of consistency for determining particle size distribution in industrial applications.