This study investigated the effect of xylanase supplementation of a plant protein-enriched diet on the growth performance and intestinal immune function of grass carp grow-out (Ctenopharyngodon idella). Five hundred forty healthy grass carp grow-out (232.74 +/- 0.12 g) were fed diets containing graded levels of xylanase for 60 days. The fish were then challenged with Aeromonas hydrophila for 14 days. The results indicated that xylanase supplementation (a) improved fish growth performance and optimized the intestinal microflora; (b) increased lysozyme and acid phosphatase activities, complement and immunoglobulin M levels and antibacterial peptide hepcidin mRNA levels, thereby enhancing fish intestinal immune function; and (c) attenuated intestinal inflammation associated with the inhibition of the NF-kappa B signalling pathway and activation of the TOR signalling pathway, resulting in the down-regulation of pro-inflammatory cytokines [except IL-1 beta and IL-12p40 in the distal intestine (DI) and IL-6 in the proximal intestine (PI)] and up-regulation of anti-inflammatory cytokines [except IL-4/13B and TGF-beta 1 in the mid intestine (MI) and DI]. Finally, on the premise of a basal diet containing 45.8 g/kg araboxylans (AXs), the appropriate supplementation levels based on the ability to improve per cent weight gain (PWG) and prevent enteritis were estimated to be 1,527 and 1,608 U/kg, respectively.