Seasonally ice-covered lakes are vital ecosystems in cold and arid regions, playing a key role in the global carbon cycle. However, the dynamics of dissolved organic matter (DOM) during freeze-thaw processes and its interaction mechanisms with phytoplankton in these lakes under the context of salinization remain unclear, constraining an accurate assessment of the evolution of their carbon sink function. In this study, four lakes with distinct salinization gradients in the cold and arid regions of China were selected as natural laboratories. Through systematic sampling during the ice-covered period, we integrated physicochemical parameters, dissolved organic matter (DOM) optical properties, and phytoplankton community data to reveal the coupling mechanisms of the DOM-phytoplankton system under salinization regulation. The results indicate that: (1) DOM in ice-covered lakes is shaped by both its initial state and under-ice dynamic processes. With increasing salinity, DOM composition exhibits characteristics of lower aromaticity and lower humification. (2) Salinization is the core driver shifting the mechanisms of phytoplankton community assembly, with an independent explanation rate of 18.71% for community variation, far exceeding that of nutrients (6.89%) and DOM (4.93%). (3) In high-salinity environments, environmental filtering dominates, with communities primarily composed of mixotrophic algae like Chroomonas, displaying a "broad niche, low diversity" pattern (mean B = 0.76). In low-salinity environments, competition predominates, where species coexistence is achieved through niche differentiation, exhibiting a "narrow niche, high diversity" pattern (mean B = 0.33). Salinization stress drives the phytoplankton system from a "high redundancy-stable" to a "low redundancy-high risk" transition, thereby altering the fate of carbon cycling during the ice-covered period. As climate warming exacerbates ice regime instability and salinization of inland waters, the systematic shifts in lake DOM and phytoplankton communities not only signify biodiversity loss but also herald a fundamental restructuring of carbon cycling patterns. Our findings provide theoretical support for predicting the future responses of stressed ecosystems, while offering a scientific basis for the ecological management and carbon sink function assessment of lakes in cold and arid regions.