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Chitosan (CTS) and chitosan oligosaccharides (COS) have been widely applied in food industry due to their bioactivities and functions. However, CTS and COS with positive charges could interact with proteins, such as whey protein isolate (WPI), influencing their digestion. Interaction among CTS/COS, FUC, and WPI/enzymes was studied by spectroscopy, chromatography, and chemical methods in order to reveal the role of FUC in relieving the inhibition of protein digestibility by CTS/COS and demonstrate the action mechanisms. As shown by the results, the addition of FUC increased degree of hydrolysis (DH) and free protein in the mixture of CTS and WPI to 3.1-fold and 1.8-fold, respectively, while raise DH value and free protein in the mixture of COS and WPI to 6.7-fold and 1.2-fold, respectively. The interaction between amino, carboxyl, sulfate, and hydroxyl groups from carbohydrates and protein could be observed, and notably, FUC could interact with CTS/COS preferentially to prevent CTS/COS from combining with WPI. In addition, the addition of FUC could also relieve the combination of CTS to trypsin, increasing the fluorescence intensity and concentration of trypsin by 83.3 % and 4.8 %, respectively. Thus, the present study demonstrated that FUC could alleviate the inhibitory effect of CTS/COS on protein digestion. [Display omitted] • FUC alleviated the negative impact of CTS and COS on protein digestibility. • CTS/COS interacted with FUC preferentially and thus WPI was released. • Electrostatic interaction occurred between -NH 3 + of CTS/COS and -SO 4 2− of FUC. • FUC could hinder the combination of CTS and trypsin. [ABSTRACT FROM AUTHOR]