To study the growth and immune performance in grass carp under three different feeding conditions, the grass carp were divided into 3 groups (group A, group B and group C), group A fed with fodder, group B fed with Guimu No. 1 hybrid grass in the morning and fodder in the afternoon, group C fed with Guimu No. 1 hybrid grass for 8 months. The results showed that grass carp fed with fodder has grown fast, of which the weight is significantly higher than those were fed with fresh grass (P>0.05); their body length was also greater than those fed with fresh grass, of which the difference showed statistical significance compared to grass carp only fed with grass (P<0.05), but no statistical significance compared to grass carp fed with fodder combined with fresh grass (P>0.05). The development of immune organs of grass carp fed with fresh grass was better than those in other groups, their head–kidney index was significantly higher than those of grass carp only fed with fodder (P>0.05), their spleen index was higher than those of grass carp fed with fodder or fodder combined with fresh grass, and the difference reached significant level (P<0.05) compared to the grass carp fed with fodder. The histological features of immune organ showed that the development of the thymus, head kidney, spleen of the fresh group is the best among all groups. In conclusion feeding fodder promotes the growth of the grass carp, but inhibits the development of the immune organ.