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Understanding the fundamental light-sample interaction process is a crucial step toward the development of vibrational spectroscopy to determine fruit texture (i.e., firmness). This study aimed to investigate the effect of pectin constitution, including total pectin, water-soluble pectin, protopectin contents, and protopectin index (<i>PI</i>), on the optical properties and firmness of ‘Baifeng’ and ‘Xiahui 8’ peach flesh at the different softening degrees during postharvest storage of 6 days at 20 °C. The firmness of ‘Baifeng’ and ‘Xiahui 8’ peaches significantly (<i>p</i> < 0.05) changed with a decreasing rate from 90.3% to 92.2%. Peach firmness of these two cultivars correlated well with <i>PI</i> contents (<i>r</i> > 0.912) and showed good internal correlations with optical scattering properties. The light absorption coefficient (<i>μ_a</i>) and reduced scattering coefficient (<i>μ’_s</i>) at 600–1600 nm were measured using a single integrating sphere system combined with an inversion algorithm. This relationship of <i>μ_a</i> and <i>μ’_s</i> with peach firmness and pectin constitution was first analyzed. Notably, the specific <i>μ’_s</i> at 660 nm, 950 nm, 1203 nm, and 1453 nm showed a satisfactory prediction of peach firmness and <i>PI</i> of ‘Xiahui 8’ (<i>R</i>² ≥ 0.926) and ‘Baifeng’ peaches (<i>R</i>² ≥ 0.764), respectively. Furthermore, the prediction models were established based on partial least squares regression coupled with optical properties, and considerable prediction performances were obtained for tissue firmness (<i>R_p</i>² ≥ 0.863) and <i>PI</i> based on <i>μ’_s</i> (<i>R_p</i>² ≥ 0.802). Consequently, these results further verified that the spectroscopic prediction model for peach firmness could be related to the high correlations between <i>PI</i> in tissues and their optical scattering properties. Future research interests could include the development of optical absorption and scattering sensors for rapid and efficient determination of peach firmness.