Objectives To explore the humoral immune function changes in old patients with community acquired pneumonia (CAP) in the high altitude localities. Methods The serum IgG, IgA and IgM levels in old patients with CAP and 215 cases of healthy control people were examined within 24h, at 4 day,7 day,10 day, 15 day, 18 day and 23 day. Results There were no significant changes on the serum IgG, IgA and IgM levels within 24 h in patients with CAP as compared with those of healthy control group（P＞0.05）. The serum IgG and IgA levels were significantly reduced at 4 day as compared with those of healthy control group （P＜0.01）, and continued to decline to 15 day. The serum IgG and IgA levels began to increase on 18 day. The serum IgG and IgA levels returned to normal levels on 23 day to compared with those of healthy control（P＞0.05）. Conclusion The serum IgG and IgA levels in old patients with CAP in high altitude area are decreased. This immune function changes with a long duration are closely related to the disease outcome. The immune therapy may shorten the course of disease, improve the prognosis and reduce mortality. It might have an important significance in the prevention and treatment of the elderly CAP.