The present study aimed to investigate the biological activities of Astragalus mongolicus (AM) water extract in mice. Mice intraperitoneally received D-gal 300 mg/kg, and orally administered with AM water extract (6, 12 or 24 g/kg) once daily for 56 days, the results of ethological examination exhibited that D-gal can weaken spatial memory function of mice in the Morris water maze test; further analysis demonstrated that the capacity of oxidative stress mediated by oxidase was enhanced in D-gal-injected mice. The decreased activities of SOD and GSH-Px and the increased activity of CAT could be found in hippocampus induced by D-gal. Based on the results above, we inferred and explored enhanced memory and physique abilities of AM water extract using three mice models (water maze test, swim test and beheaded breathing test), the results exhibited that AM water extract could prolong the swimming time, the breathing time and survival time. This work indicated that AM water extract was very helpful to organism, and the protective effect may relate to oxidative stress.