Objective To investigate the effect of nutritional status on the severity and clinical prognosis of community acquired pneumonia (CAP) in the elderly. Methods One hundred and fifty elderly CAP patients hospitalized in our department from January 2018 to September 2018 were selected. The severity of pneumonia was assessed with pneumonia severity index (PSI) within 24h after admission, and according to the severity of pneumonia, the patients were divided into non-high-risk pneumonia group (n=99) and high-risk pneumonia group (n=51). The nutritional status was evaluated with nutritional risk screening 2002 (NRS2002) scale within 72h. Their basic data, laboratory indices and clinical prognosis were recorded and compared between the 2 groups. SPSS statistics 22.0 was used for statistical analysis. According to the data type, Student′s t test, rank sum test or Chi-square test was used for comparison between groups. Multivariate logistic stepwise regression analysis was used to investigate the influencing factors of high-risk pneumonia in CAP patients. Results Multiple logistic regression analysis showed that increased nutritional risk score (OR=0.481,95%CI 0.231-0.999, P=0.049) and red blood cell volume distribution width (OR=2.400,95%CI 1.302-4.425, P=0.005), heart failure (OR=7.854,95%CI 1.086-56.784, P=0.041), oral missing teeth/denture (OR=19.84,95%CI 1.366-288.219, P=0.029), blood urea (OR=2.050,95%CI 1.219-3.446, P=0.007) and lower Barthel score for ability of daily life score (OR=0.452, 95%CI 0.237-0.862, P=0.016) were influencing factors for high-risk pneumonia in elderly CAP patients (P<0.05). In all these patients, there were significant differences in 30-day clinical prognosis (P=0.016) and discharge outcome (P=0.012) between those with nutritional risk and those with normal nutrition. The prognosis of those with nutritional risk was worse. Conclusion The elderly CAP patients are prone to high-risk pneumonia and poor prognosis. Therefore, special attention should be paid to the elderly with nutrition risk, cardiac insufficiency, oral hygiene and decline of daily living ability.