Plateau lakes, owing to their distinctive seasonal ice cover, play a special role in global greenhouse gas (GHG) emissions, characterized by under-ice accumulation in winter and pulsed release during ice-off. Focusing on Lake Wuliangsuhai in Inner Mongolia, this study resolves the spatiotemporal dynamics of CH? and N?O in the overlying water during the freeze-thaw period and their correlations with environmental factors. Based on freeze-thaw-induced biogeochemical processes in sediments, we further examine variations in carbon and nitrogen nutrients in the overlying water and in sedimentary electron acceptors, thereby elucidating the mechanisms governing GHG storage and release during freeze-thaw. Results demonstrate that formation of an ice cover drives cumulative increases in dissolved CH? in the overlying water. The highest surface-water CH? concentration occurred in vegetated zones during the stable frozen stage (93,382.76 nmol/L), whereas the peak N?O concentration was recorded in occurred in non-vegetated zones at the end of thawing (181.93 nmol/L). In contrast to N2O, whose behavior is largely controlled by internal sedimentary processes and mass-transfer limitations, CH4 responds more sensitively to variations in DO and ORP of the overlying water. Across the freeze-thaw cycle, total organic carbon (TOC) and total nitrogen (TN) accumulated during freezing but declined during thawing due to decomposition. Inorganic nitrogen species (NH??-N, NO??-N, NO??-N) shifted dynamically and reciprocally, consistent with active nitrification and denitrification. Carbon and nitrogen migration was jointly regulated by sediment biogeochemistry and redox conditions imposed by freeze-thaw. Sedimentary organic-matter degradation to displayed vertical stratification: nitrate/iron reduction dominated in upper-middle layers, while sulfate reduction prevailed at depth. Methanogenesis accompanied these pathways, with CH? migrating upward across the sediment-water interface and ultimately accumulating beneath the ice.