Thirty SD rats were randomly divided into five groups: normal group, model group and low, medium and high dose L-theanine group according to their body weight. The aging rat model was induced by daily subcutaneous injection of 200 mg/kg d-galactose physiological saline solution to investigate the regulation of L-theanine on the activity of alanine transaminase(ALT) and aspartate transaminase(AST), the contents of advanced glycation end products(AGEs) and inflammatory mediators in the liver. D-galactose-induced aging rats were treated with physiological saline, low, medium and high doses(100, 200, 400 mg/kg) of L-theanine physiological saline solution for 8 weeks. The organ index of spleen and thymus, the activity of ALT and AST, the contents of AGEs and the mRNA and protein levels of inflammatory mediators were measured after the treatment. The results showed that L-theanine could increase the spleen index and thymus index of d-galactose-induced aging rats, inhibit the up-regulation of liver ALT and AST activity, down-regulate the content of AGEs in the liver, and at the same time inhibit mRNA levels and protein levels of inflammatory mediators such as tumor necrosis factor-α(TNF-α), tumor necrosis factor receptor-1(TNFR1), interleukin 1 beta(IL-1β), interleukin-1 receptor(IL-1R), interleukin 6(IL-6), inducible nitric oxide synthase(iNOS), intercellular adhesion molecule -1(ICAM-1) and vascular cell adhesion molecule-1(VCAM-1), to maintain inflammatory internal environment stability in the liver. Among them, L-theanine in the middle and high dose groups had a better protective effect on liver of d-galactose-induced rats. In summary, L-theanine can improve immunity, inhibit the level of liver AGEs and inflammatory mediators, maintain homeostasis in the liver, and have the potential to prevent liver inflamm-aging.