Objective To investigate the ratio, risk factors and prognosis of very old patients suffering from hospital-acquired pneumonia (HAP) complicated with sepsis. Methods A total of 156 very old HAP patients (≥80 years old) admitted in our hospital from January 2017 to December 2020 were recruited in this study. They were divided into sepsis group (n=72) and non-sepsis group (n=84). The clinical outcomes were observed, with the indicators such as in-hospital mortality, 28-day mortality and 30-day readmission rates after discharge. SPSS 22.0 software was used for data analysis. Data comparison between two groups was perfomed using t test, Fisher test or χ² test depending on data type. Logistic regression analysis was adopted to analyze the risk factors. Results Malnutrition (OR=2.419,95%CI 1.151-5.084, P=0.020), diabetes mellitus (OR=2.335,95%CI 1.007-5.415, P=0.048) and chronic renal disease (OR=7.602,95%CI 2.365-24.429, P=0.001) were independent risk factors for sepsis. There was no difference in 30-day readmission rate between the 2 groups, while the in-hospital and 28-day mortalities were significantly higher in the sepsis group than the non-sepsis group [18(25.0%) vs 8(9.5%), 21(29.2%) vs 8(9.5%), both P<0.05]. The independent risk factors of in-hospital mortality were malnutrition (OR=3.331,95%CI 1.010-10.851, P=0.020), chronic heart failure (OR=5.904,95%CI 1.947-17.903, P=0.048) and chronic renal disease (OR=1.973,95%CI 1.243-13.619, P=0.001). Conclusion Malnutrition, diabetes mellitus and chronic renal disease may be associated with the occurrence of sepsis in the very old HAP patients, and the complication greatly increases the mortality in the population.