Objective To investigate the relationship between the red blood cell distribution width (RDW) and the severity of community-acquired pneumonia (CAP). Methods The clinical data of 285 adult patients with CAP admitted from November 2014 to August of 2017 were analyzed retrospectively. The severity of CAP was evaluated by pneumonia severity index (PSI) score. Meanwhile, 60 cases with qualified medical examination were collected as a healthy control group. The distributions of PSI score, RDW, procalcitonin (PCT), C-reactive protein (CRP) and neutrophil percentage (NEU%) were described in the patients with different risk degree. The correlation analysis of various indicators were analyzed by Spearman correlation. The threshold of RDW(%) was calculated through the construction of the general linear regression equation. The risk factors of PSI score were analyzed with multiple linear regression. Results The higher the risk stratification, the higher the distribution of PSI scores, RDW, PCT, CRP and NEU% were. RDW was positively correlated with PCT, CRP, and NEU% (r values were 0.417, 0.252, 0.318, respectively, P<0.05). PSI score was positively correlated with RDW, PCT, CRP, and NEU% (r values were 0.537,0.598, 0.557, 0.482, respectively, P<0.05). RDW was positively correlated with PSI score (r=0.537, P<0.05). The thresholds of RDW(%) were 14.514 and 19.041. Multiple linear regression showed that RDW, PCT, CRP and NEU% were all influential factors of PSI scores and explained 46.9% of the total mutation rate. Conclusion RDW is correlated with the severity of CAP, and can predict the severity of CAP.