Laser-Induced Breakdown Spectroscopy (LIBS) technology can directly analyze online slurry. However, due to the complex matrix composition of slurry, variations in solid particle concentration and particle size can affect the spectral signal, making the relationship between the spectral signal and mineral chemical composition difficult to characterize. Therefore, studying how slurry solid phase concentration and particle size influence the spectral signal is crucial. In this study, SiO₂ powder of various particle sizes was mixed with different amounts of water to create simulated slurry samples with varying solid-phase concentrations and particle sizes. LIBS spectral signal acquisition and data analysis were conducted to systematically investigate the impact of solid-phase concentration and particle size on the intensity of characteristic spectral lines of elements in both the solid and liquid phases. Firstly, the Pearson coefficient was used to quantitatively evaluate the impact of solid-phase concentration and particle size on the LIBS spectral signal. The results showed that, under the same particle size, the correlation coefficient between solid-phase concentration and the full spectrum value and the intensity of characteristic spectral lines of various elements was mostly above 0.9, indicating that overall spectral intensity increases with the solid-phase concentration. Under the same solid-phase concentration, the correlation coefficient between the principal components of particle size and the intensity of silicon element characteristic spectral lines was around 0.99, indicating that the characteristic spectral lines of elements in solid particles weaken as the particle size increases. Furthermore, a model was established to describe the relationship between spectral intensity solid-phase concentration and particle size when both parameters change simultaneously. The model's goodness of fit (R²) was used to analyze the impact of solid-phase concentration and particle size on the LIBS spectrum. The results indicated that only the solid-phase particle size representation model for silicon element characteristic spectral line intensity achieved goodness of fit above 0.9, suggesting that neither solid-phase concentration nor particle size alone can fully and accurately reflect changes in the intensity of characteristic spectral lines of elements in the samples. The model that simultaneously characterizes spectral intensity based on both solid-phase concentration and particle size achieved a goodness of fit exceeding 0.9 for the characteristic spectral lines of all elements, indicating that the influence of solid-phase concentration and particle size on the intensity of characteristic spectral lines is coupled and requires comprehensive characterization through multi-source information integration. These findings provide a systematic analytical foundation for further research on enhancing the stability and accuracy of quantitative LIBS analysis based on multi-source information fusion.