To investigate the relationship between tongue coating changes and tongue microbiota in colorectal cancer(CRC) patients during postoperative chemotherapy.Methods:Ninety-nine CRC patients undergoing chemotherapy were recruited from the Department of Integrative Oncology，The First Affiliated Hospital of Harbin Medical University，between June 2023 and April 2024.Tongue characteristics were recorded using the DS01-B tongue diagnostic instrument.Patients were categorized into three groups based on tongue coating type：thin white coating(BBT)，thick white coating(BHT)，and greasy white coating(BNT).Each group was further divided according to chemotherapy cycle：BBT at cycles 1 to 4(A1) and 5 to 8(A2); BNT at cycles 1 to 4(B1) and 5 to 8(B2); BHT at cycles 1 to 4(C1) and 5 to 8(C2).Tongue coating samples were collected from 48 patients and subjected to high-throughput 5-region 16S rDNA sequencing to analyze microbial composition.Results:BBT，BHT，and BNT were the predominant tongue coatings observed during chemotherapy in CRC patients.The top five genera were consistent across the three coating types，with similar relative abundances.As chemotherapy progressed，there was a tendency for tongue coatings to shift from greasy to thin types，correlating with changes in microbial abundance.Characteristic genera were identified for each tongue coating：Porphyromonas and Paracoccus for BBT，Megasphaera for BHT，and Oribacterium，Proteus，and Weissella for BNT.In long-term chemotherapy，BBT samples showed higher abundance of Proteobacteria and Clostridium than that in short-term chemotherapy(P<0.05)，BHT samples exhibited increased Actinomyces(P<0.05)，and BNT samples had elevated Ciliophora(P<0.05).Conclusion:The tongue microbiota of CRC patients remains relatively stable during chemotherapy，with BBT，BHT，and BNT as the predominant coating types.While microbial community structure is similar across coating types，relative abundance differs.Tongue coating shows a changing trend with prolonged chemotherapy，accompanied by an increase in pathogenic microbial abundance，particularly in BBT.