Abstract: In order to study the effects of fermented Astragalus on spleen immune function in broilers, in this experiment, 120 1-day-old 817 healthy male broilers were selected and divided into blank control group, test groups Ⅰ, Ⅱ and Ⅲ using the one-way complete randomization method, with three replicates of 10 chickens in each group. The broilers in the blank control group drank tap water without adding any drugs or additives, while 0.5‰ plant Lactobacillus, 0.5‰ Astragalus aqueous extract and 0.5‰ fermented Astragalus aqueous extract were added to tap water in test groups Ⅰ, Ⅱ and Ⅲ, respectively, for a period of 21 d. On day 21, the spleens of broilers in each group were collected, weighed and the spleen indexes were measured; and the morphological structure changes of the spleen tissue and the proliferation of spleen lymphocytes were detected by paraffin section-hematoxylin eosin staining and immunohistochemical staining of rabbit-derived proliferating cell nuclear antigen（PCNA）, respectively. The results showed that compared with the blank control group, the spleen indexes increased by 11.41% and 15.44% in groups Ⅱ and Ⅲ, respectively, but decreased in group Ⅰ; all differences were not significant（P＞0.05）. There were no significant changes in color, appearance and tissue structure of the spleen of broilers between blank control group and test groups Ⅰ, Ⅱ, Ⅲ. Compared with the control group, the spleen lymphocyte count increased by 33.53% in group Ⅲ, which was significant（P＜0.05）. The results indicated that fermented Astragalus could promote the growth and development of broiler spleen and lymphocyte proliferation, which could improve the immune function of spleen and enhance the immunity of chickens, but the specific mechanism needed to be studied in depth.