Abstract: Micro/nanoplastics (M/NPs) are widely distributed in aquatic environments, are significant threats to aquatic life as carriers of pollutants． Ultraviolet-visible (UV-Vis) spectrophotometry, due to simplicity and sensitivity, holds potential for environmental analysis． The UV-Vis absorption spectra (190-1100 nm) of polystyrene nanoplastics (PSNP, φ = 100 nm) were measured. Spectral interference from NaCl, MgCl₂ and CaCl₂ were analyzed． PSNP and these inorganic salts showed overlap in absorption in the region of 190-340 nm． Increases in Cl⁻ concentration were found to induce redshifts in resultant spectra, leading to deviations from the Lambert-Beer law, to interfer with the quantitative determination of PSNPs． Redshifts were corrected to construct a nonlinear quantitative model, with goodness-of-fit (R _\mathrmadj^2 ) for NaCl, MgCl₂, and CaCl₂ solutions improved to 0.998-0.999． Model validation using mixed systems indicated that predicted absorbance of PSNPs in NaCl solution reached (98.2 ± 3.0)% of standard values． However, when MgCl₂ and CaCl₂ concentrations approached or exceeded critical coagulation concentration (CCC), aggregation and sedimentation of PSNPs occurred, resulting in a significant decrease in absorbance． Overall, the proposed redshift-corrected nonlinear model could effectively enhance accuracy of quantitative M/NPs analysis of environmental waters．