The intensification of organic matter pollution is a critical issue in lake eutrophication research. Typically for macrophyte-dominated lakes of cold-arid regions, the increasing organic matter pollution, aggravating eutrophication status, and intensifying global changes amplify uncertainties in the evolution of lake ecosystems. This study investigates seasonal migration and transformation mechanisms of organic matter in Lake Ulansuhai, a macrophyte-dominated cold-arid eutrophic lake. Our findings revealed that Lake Ulansuhai exhibits persistently high organic matter concentrations in water and sediment, with dissolved organic matter dominating in the water column. Unlike subtropical lakes, organic matter burial in Lake Ulansuhai was significantly higher, peaking in winter, while mineralization occurred throughout spring, summer, and autumn. Summer conditions-elevated temperatures and algal-sourced organic matter-further accelerated mineralization. In addition, the accumulation of algal-sourced organic matter in the sediment and rapid consumption of algal-sourced organic matter in the water suggests an escalating trend in algal blooms. The warming trend of the area might intensify mineralization of buried organic matter through earlier ice melt and higher seasonal temperatures. These processes would in turn increase carbon emissions and promote a potential trend of transitioning to an algal-dominated turbid state. Future studies should give more focus on these accelerated organic matter cycling processes in similar cold-arid eutrophic lakes under an intensified global change trend.