To study the effect of different doses of Lactobacillus preparation on growth performance, cecal bacterial and immune organs of 817 broilers. 720 one-day old 817 broilers were divided into five groups randomly; the broilers in control group were fed with a basal diet, and Ⅰ, Ⅱ, Ⅲ, Ⅳ experimental groups were fed with the basal diet supplemented with 100, 200, 300 and 400 mg/kg Lactobacillus preparations respectively. The whole experiment period was 49 days, including two phases of 1 to 21 d and 22 to 49 d. The results showed that: (1) from 1 to 21 d, dietary supplementation of Lactobacillus preparation could increase daily weight gain and survival ratio, and lower feed conversion of broilers; specifically, effects on Ⅱ, Ⅲ and Ⅳ groups were significant（P＜0.05); from 22 to 49 d, compared with control group, the daily weight gain, feed conversion and survival ratio improve significantly with group Ⅱ（P＜0.05), and addition of 200 mg/kg Lactobacillus preparation had the best effect in the whole period. (2) Compared with control group, the number of cecal Lactic acid bacteria and Bifidobacterium had increased obviously, cecal E.coli and Salmonella had decreased obviously with each experimental groups in the whole period, and the difference between Ⅱ, Ⅲ, Ⅳ groups（P＜0.05) were significant. (3) Compared with control group, spleen index of Ⅳ group was significantly improved at 21 d（P＜0.05), spleen index of each experimental groups were significantly improved at 42 d（P＜0.05); thymus and bursa of fabricius indexes of each experimental groups were higher than that in control group, but the difference was not significant（P＞0.05). In conclusion, the biggest improvement for growth performance of 817 broilers was addition of 200 mg/kg Lactobacillus preparation, but addition of 400 mg/kg was better for balance of intestinal bacterial and immune function improvement under the condition of experiment.