Tianwen-1’s MarSCoDe payload acquires chemical compositions of Martian rocks and soils. Previous LIBS models, built with Earth-based simulated Martian spectra, face accuracy reduction due to environmental discrepancies between simulations and the real Martian surface. To address this issue, this study establishes a univariate quantitative model for major elements based on in-situ Martian MCCT LIBS spectra. The derived Root-Mean-Square Errors are comparable to those reported in previous studies, and the average elemental compositions are consistent with GRS measurements at the Tianwen-1 landing site. Based on model-derived compositions and low Chemical Index of Alteration (CIA, 21.5% ± 8.0%), most rocks and soils show overall weak chemical alteration, implying limited water–rock interactions in the Tianwen-1 landing area during the Late Hesperian to Amazonian. Nonetheless, several samples with CIA values exceeding 35% (up to 42.7%) record localized liquid water activities within the region during this period.