The invention discloses new application of protocatechuic acid in the aspect of immune function promotion of livestock, especially in the aspects of peripheral blood CD4+ / CD8+T cell dynamic balance maintenance of livestock, cytokine secreting adjustment and the like. Influences of protocatechuic acid on the immune function, especially on T cell differentiation, of livestock are evaluated, 20 mg of protocatechuic acid is applied to each kilogram of body weight, protocatechuic acid is continuously taken orally for seven days, and results show that protocatechuic acid can effectively increase the peripheral blood CD4+ / CD8+T ratio, promote cytokine secreting in thymuses and the spleen and selectively adjust related genes of the immune system. The invention further provides a livestock immune promoter containing an effective dose of protocatechuic acid. Medical auxiliary materials can be added and prepared into various preparations, such as powder, tablets, granules and oral liquid, according to a conventional preparation method in the field. The effective dose is 5-40 mg for each kilogram of body weight, and the preparation is taken in a mixing mode or with water.