〔Abstract〕 Objective To explore the effects of pituitary adenylate cyclase activating polypeptide(PACAP) and recombinant PACAP(PACAP-PTD) containing protein transduction domain on thymus function in D-galactose(D-Gal)-induced immunosenescence mice, and to clarify its role in delaying immune aging. Methods Thirty male BALB/c mice were randomly divided into control group and model group, and 5 rats in the control group were injected subcutaneously with 500 mg/kg normal saline per day. The model group was injected with the same amount of D-Gal solution. After continuous injection for 42 days, 18 rat in the model group were randomly selected as PACAP-PTD group, PACAP group and D-Gal group, 6 for each group. The PACAP group and the PACAP-PTD group were intraperitoneally injected with 12.5 ml/kg PACAP and PACAP-PTD solution, and the D-Gal group was injected with the same amount of normal saline. Real-time PCR was used to detect initial T cell markers(sjTREC) in the thymus and spleen, the expression of the thymus-associated genes LIM domain only 2(LMO2), interleukin-7(IL-7) and forkhead box protein N1(Foxn1). Flow cytometry was used to detect the phenotypic changes of T lymphocytes CD3,CD4 and CD8 in whole blood of mice. Results The content of sjTRECs in T lymphocytes of mouse thymus and spleen: D-Gal groupP<0.01). Compared with the D-Gal group, PACAP and PACAP-PTD could up-regulate the expression of IL-7 and Foxn1(P<0.01), and down-regulate the expression of LMO2(P<0.01);moreover,PACAP-PTD up-regulated the expression of Foxn1 and IL-7 more significantly than PACAP(P<0.01). Ratio of CD3+CD4+/CD3+CD8+in mouse whole blood T lymphocyte subsets: D-Gal groupP<0.01). Conclusion PACAP and PACAP-PTD significantly enhance the initial T cell production and export function of mouse thymus, keep the diversity of peripheral T lymphocyte bank, up-regulate the expression of thymus function and statE-related genes, expand the thymus microenvironment, enhance cellular immune function and restore degraded thymic function, which have the effect of delaying aging and immune protection.