Objectives The effects of adding different levels of yeast culture to the basic diet on the growth performance and immune indexes of fattening sheep were studied to provide reference for seeking products instead of antibodies with effects of promoting growth. Methods 160 healthy fattening rams aged 3-4 months with a body weight of (40±2) kg were selected and randomly divided into 4 groups using a random number table method. They were fed with experimental diets supplemented with 0(control group), 0.3%(low dose group), 0.6%(medium dose group), and 0.9%(high dose group) yeast culture in the basic diet for 90 days. The average daily intake of feed, average daily gain of weight, and the ratio of feed to weight in fattening sheep were determined. The content of IgG, IgA, and IgM in the venous serum of fattening sheep were measured as well. Results Compared with the control group, the average daily intake of feed, average daily gain of weight of fattening sheep in the low dose, medium dose, and high dose groups significantly increased, while the ratio of feed to weight significantly decreased. The content of IgG, IgA, and IgM in the venous serum of fattening sheep in the low dose, medium dose, and high dose groups significantly increased. Among them, the experimental group with 0.6% yeast culture added had an average daily gain of weight of 176.56 g for fattening sheep, and the content of IgG, IgA, and IgM in the venous serum of fattening sheep was 29.25, 191.25, and 2 165.49 μg/mL, respectively. Conclusions Adding yeast culture to the diet can improve the growth performance and enhance the immune capacity of fattening sheep to a certain extent, with the best effect of adding 0.6% yeast culture to the diet.