Objective：To investigate the relationship between dynamic blood phosphorus level and the severity of sepsis in elderly patients and assess its prognostic value. Methods：Elderly septic patients admitted to the ICU of the Geriatric Hospital Affiliated to Nanjing Medical University from September，2014 to June，2018，who met the inclusion and exclusion criteria，were involved for a retrospective analysis. The general data and the serum phosphorus levels of the patients were collected on the 1st，3rd and 7th days of ICU entry. According to the severity of sepsis，the patients were divided into sepsis group，severe sepsis group and septic shock group. The differences of dynamic blood phosphorus levels among the three groups were evaluated. According to the 28-day prognosis，the patients were divided into survival group and death group，and the difference of dynamic blood phosphorus levels between the two groups was evaluated. The receiver operating characteristic curve（ROC）of APACHE Ⅱ score and serum phosphorus levels on the 1st，3rd and 7th days of ICU entry was used to evaluate the prediction for 28-day prognosis. Results：A total of 104 elderly patients with sepsis were involved in this study. No significant difference in blood phosphorus levels on the 1st day of ICU entry was found among the three groups with different severity of the disease（P > 0.05）. However，the blood phosphorus levels on the 3rd and 7th days in the septic shock group were significantly lower than those in the sepsis and severe sepsis groups（P < 0.05）. There was no significant difference in the blood phosphorus levels on the 1st and 3rd day of ICU entry between the survival group and the death group（P > 0.05），and the blood phosphorus level on the 7th day in the death group was significantly lower than that in the survival group（P < 0.05）. ROC curve showed that APACHE Ⅱ score（AUC=0.679）and blood phosphorus level（AUC=0.641）on the 7th day of ICU entry had higher predictive value for the prognosis of elderly septic patients than the blood phosphorus levels on the 1st day of ICU entry（AUC=0.601）and the 3rd day of ICU entry（AUC=0.616）. Conclusion：Dynamic blood phosphorus level is of some significance in indicating the severity and prognosis of sepsis in elderly patients. The blood phosphorus level on the 7th day of ICU entry has better predictive value in the prognosis of sepsis in elderly patients，which is significantly lower in the death group than in the survival group.