Objective To analyze the related risk factors affecting the prognosis of elderly patients with severe community acquired pneumonia (SCAP). Methods A retrospective study method was conducted; the elderly (≥ 75 years old) patients with SCAP treated in the First Affiliated Hospital of Hainan Medical College from January 2015 to January 2019 were enrolled. The general data of patients were collected, including sex, age, oxygenation index (PaO2/FiO2), involved organs, presence or absence of following diseases or treatment: damage in multiple lung lobes, septic shock, basic diseases (cardiovascular disease, chronic lung disease, diabetes, hypertension, and cerebrovascular disease), invasive mechanical ventilation, ventilator-associated pneumonia (VAP), misinhalation event, hyponatremia, respiratory acidosis, hypoproteinemia, intubation times, total mechanical ventilation time, etc. According to the prognosis, the patients were divided into a death group and a survival group. The general data were compared between the two groups with different prognoses. Single factor analysis was carried out by selecting variables. The indicators with statistical significant differences in the results of univariate analysis were introduced into the multivariate Logistic regression analysis to analyze the related risk factors affecting the prognosis of elderly patients with SCAP. The receiver operating characteristic (ROC) curve was drawn to analyze the predictive values of risk factors in the patients with SCAP. Results A total of 112 patients were included, 33 died, and the mortality rate was 29.46%. Univariate analysis showed that the following factors were higher in the death group than those in the survival group: organ involvement >2 [69.70% (23/33) vs. 35.44% (28/79)], lung lobe damage ≥ 3 [75.76% (25/33) vs. 51.90% (41/79)], invasive mechanical ventilation [72.73% (24/33) vs. 32.91% (26/79)], diabetes [30.30% (10/33) vs. 12.66% (10/79)], intubation times ≥2 [57.58% (19/33) vs. 48.10% (38/79)], hypoproteinemia [75.76% (25/33) vs. 41.77% (33/79)], hyponatremia [72.73% (24/33) vs. 48.10% (38/79)], respiratory acidosis [66.67% (22/33) vs. 44.30 %(35/79)] and total mechanical ventilation time ≥ 15 days [69.70% (23/33) vs. 40.51 (32/79)]; the factors in the death group lower than those in the survival group were: septic shock [3.03% (1/33) vs. 17.72% (14/79)], chronic lung disease [6.06% (2/33) vs. 25.32% (20/79)] and PaO2/FiO2 [mmHg (1 mmHg = 0.133 kPa): 102.89±14.78 vs. 109.56±14.08],the differences were statistically significant (all P < 0.05); there were no significant differences in gender, age, cardiovascular disease, hypertension, VAP, misinhalation events and cerebrovascular disease between the two groups (all P > 0.05). Multivariate Logistic regression analysis showed that diabetes mellitus [odds ratio (OR) = 1.074, 95% confidence interval (95%CI) = 1.017-1.287, P =0.045], septic shock (OR = 2.765, 95%CI = 1.083-3.411, P = 0.047), hyponatremia (OR = 1.792, 95%CI = 1.128-1.417, P = 0.006), hypoalbuminemia (OR = 2.187, 95%CI = 1.872-5.462, P = 0.046), invasive mechanical ventilation (OR = 5.870, 95%CI = 2.324-23.796, P = 0.001), respiratory acid poisoning (OR = 2.934, 95%CI = 2.454-7.275, P = 0.043), time of mechanical ventilation (OR= 1.986, 95%CI = 2.467-3.483, P = 0.034), number of intubation (OR = 6.760, 95%CI = 2.116-24.696, P = 0.001), PaO2/FiO2 (OR = 1.981, 95%CI = 1.006-1.417, P = 0.007), organ involvement > 2 (OR = 2.924, 95%CI = 2.534-6.285, P = 0.048), chronic lung disease (OR = 2.887, 95%CI = 1.487-3.483, P = 0.039), and lung lobe damage≥3 (OR = 2.754, 95%CI = 1.131-1.798, P = 0.045) were independent risk factors affecting the prognosis of elderly patients with SCAP. ROC analysis showed that hyponatremia, hypoalbuminemia, invasive mechanical ventilation, total mechanical ventilation time, PaO2/FiO2, organ involvement > 2, damage of lung lobes ≥ 3, had predictive values for the prognosis of SCAP [the areas under ROC curve (AUC) were 0.377, 0.267, 0.301, 0.646, 0.650, 0.329, and 0.381, respectively, all P < 0.05]. Conclusions Underlying disease, invasive mechanical ventilation, respiratory acidosis, total mechanical ventilation time, PaO2/FiO2, intubation times ≥ 2, chronic lung disease and lung damage≥ 3 lobes are the independent risk factors for the prognosis of elderly patients with severe community acquired pneumonia. Clinical treatment should focus on the above aspects to minimize the mortality of patients.