Xiyanping injection (andrographolide sulfonate) has shown clinical effects on community acquired pneumonia. However, there is little known about the effectiveness and costs of combining Xiyanping injection with conventional treatment on adult community acquired pneumonia in daily practice. The aim of this study was to evaluate the cost-effectiveness of combining Xiyanping injection with conventional treatment for treatment of adult community acquired pneumonia by comparing with conventional treatment from a societal perspective. Using retrospective cohort method, this study demonstrates that Xiyanping injection combined with conventional treatment is superior to conventional treatment for patients using cephalosporins and antibiotics under the effectiveness index of length of hospital stay and is more cost-effective.