Introduction Plastic mulch is widely used in agriculture as it increases soil temperature and reduces water evaporation, thereby enhancing water use efficiency and crop yields. However, prolonged use of plastic mulch leaves residues that negatively impact soil structure, impede water and salt movement, and reduce cotton yields. While the impact of mulch contamination on water-salt distribution is recognized, the specific mechanism by which increasing the C/N ratio improves this distribution remains unclear. We hypothesize that by raising the C/N ratio, it is possible to enhance the water-salt environment, thus stabilizing lint cotton yields.Methods A two-year field experiment (2024-2025) was conducted in Xinjiang, China. Three irrigation levels were established based on crop evapotranspiration (ETc): 0.6 ETc (I1, severe deficit), 0.8 ETc (I2, mild deficit), and 1.0 ETc (I3, full). Four C/N ratio treatments were applied: 0:1 (F0), 3:1 (F1), 6:1 (F2), and 9:1 (F3).Results Across irrigation levels, increasing the C/N ratio of the fertilizer significantly enhanced soil water-holding capacity, reduced salt accumulation, and improved water and salt uniformity. Yield under C/N 6:1 (F2) was 7.9% higher on average than other treatments. Under different C/N ratios, mild deficit irrigation (I2) maintained higher soil moisture uniformity and desalination rate(Rs); soil salinity was 5.4% lower than under I1 but 9.5% higher than under I3; seed cotton yield was 17.0% and 0.8% higher than I1 and I3, respectively. Compared with I3F0, I2F2 increased soil moisture by 4.2%, moisture uniformity by 4.1%, reduced salinity by 1.1%, and raised yield by 9.1%. Correlation analysis showed desalination rate, moisture content, and its uniformity coefficient were significantly positively correlated with lint yield, while soil salinity was significantly negatively correlated.Discussion A C/N ratio of 6:1 effectively improves the water-salt balance in plastic mulch-contaminated cotton fields by optimizing soil water and salt distribution, thereby increasing yield. This provides mechanistic insight and practical strategies for water-saving and salt-suppression management in arid regions.