Objective To observe the effect and mechanism of Qiangji Jianli Formula（Herbal formula for strengthening muscles and bones）on the thymus of rat models with spleen-kidney deficiency syndrome.Methods Seventy-two SD rats were randomized into the following five groups:the normal control group（n=10）

model group（n=16）

Qiangji Jianli Formula group（n=15）

Qiangji Jianli Formula with the Huangqi（Radix Astragali seu Hedysari）dosage reduced group（n=15）

and Huangqi（Radix Astragali seu Hedysari）group（n=16）.The spleen deficiency models were established by using Dahuang（Radix et Rhizoma Rhei）for 14 days

based on which

the kidney deficiency models were established by injectng cortisol for 10 days.At the same time

the distilled water

Qiangji Jianli Formula

Qiangji Jianli Formula with Huangqi reduced in dosage

and Huangqi were administered by gavage respectively.After the administration for 24 days in succession

the thymus of rats of all groups was obtained to observe the morphological changes under optical microscope and to determine the expression of proliferating cell nuclear antigen（PCNA）with the immunohistochemical method.Results Compared with the normal control group

the body weight of the model group was reduced significantly（P<0.01）

and that of all the treated groups was increased（P<0.05 or P<0.01）.The thymus of the model group was damaged

while the Qiangji Jianli Formula group and Huangqi group got their damaged thymus repaired.Compared with the normal control group

PCNA expression in the model group was lowered and the rate of positive area was significantly lowered（P<0.01）

whilst Qiangji Jianli Formula group got its PCNA expression of thymus increased（P<0.05）.Conclusion Qiangji Jianli Formula can promote the proliferation of cells of thymus of model rats with spleen-kidney deficiency syndromes

and protect the damaged thymus.Its mechanism of strengthening the spleen and replenishing qi is related to the increase of PCNA expression.