This study examined how a sudden drop in temperature affects the skin mucus barrier in grass carp.The findings indicated a reduction in the total epidermal mucous cells in grass carp, accompanied by a thinning and potential detachment of the mucus layer, in response to a sudden decrease in temperatureNotably, type II mucous cells exhibited heightened sensitivity to temperature fluctuations.Concurrently, immune-related enzyme activity within the mucus decreased.High-throughput sequencing and metabolomic anal. revealed that a sudden drop in temperature reduced the abundance and diversity of skin symbiotic bacteria, increased disease-associated pathogens, and decreased beneficial bacteria, with Nocardia increasing to 34.03% and Cetobacterium decreasing to 0.28%, increasing disease risk in grass carp.Furthermore, the sudden drop in temperature affected the metabolic state of grass carp and skin symbiotic bacteria, decreasing nucleotide sugars, nucleotide synthesis precursor, and lysine biosynthesis precursors and increasing purine catabolites, stress regulators, and tryptophan metabolites in mucus.This disturbs energy metabolic, nucleotide levels, protein and cellular structural component synthesis, and stress states, upsetting mucous layer balance.In addition, we discovered six metabolites that are sensitive to temperature changes and could serve as potential biol. indicators of temperature correlation.In summary, a sudden drop in temperature weakens the protective function of the skin mucus barrier, causing an imbalance in the microbial structure and metabolic function.This study aids in understanding the role of mucus in innate immunity and provides early warnings of danger to fish.