The present work evaluates the effects of various levels of dietary choline on immune parameters, immune-related gene expression and protection against Aeromonas hydrophila (AH) in juvenile Jian carp (Cyprinus carpio var. Jian). Fish were fed with six different experimental diets containing graded levels of choline at 165 (choline-deficient control), 310, 607, 896, 1167 and 1820 mg kg−1 diet for 65 days. At the end of the feeding trail, Fish were challenged with AH and mortalities were recorded over 17 days. Dietary choline significantly enhanced spleen and head kidney weights, spleen index, red blood cell and white blood cell counts, and intestinal Lactobacillus counts of juvenile Jian carp; whereas, intestinal Escherichia coli and A. hydrophila counts decreased. Moreover, the post-challenge survival rate, leucocyte phagocytic capacity, serum lysozyme and acid phosphatase activities, hemagglutination titer, complement 3 and 4 contents, immunoglobulin M content, and anti-AH antibody titer were significantly enhanced by choline and the lowest in choline-deficient group, while serum total iron-binding capacity was the highest in choline-deficient group. The relative gene expressions of interleukin 10 in spleen and head kidney, target of rapamycin (TOR) in spleen and eIF4E-binding protein2 (4E-BP2) in head kidney significantly increased with increasing of dietary choline up to a certain point. However, the relative gene expressions of interleukin 1β, tumor necrosis factor α and transforming growth factor β2 in spleen and head kidney, TOR in head kidney and 4E-BP2 in spleen significantly decreased. In conclusion, dietary choline improved disease resistance, enhanced the immune function, and regulated immune-related gene expression of juvenile Jian carp.