Objective To explore the dynamic changes of procalcitonin (PCT) and D-dimer (D-D) and investigate their values in the prognosis for severe pneumonia in the elderly patients. Methods A total of 78 elderly patients with severe pneumonia admitted in our hospital from January 2019 to June 2020 were recruited in this study. Their clinical outcome, PCT and D-D levels before treatment on the 3rd and 7th days of treatment, and immediately before out of ICU (or death) were recorded, and change rate of PCT (PCTc) and D-D (D-Dc) were calculated. Their score of Acute Physiology Age Chronic Health Evaluation Ⅱ (APACHE Ⅱ) were also recorded. Receiver operating characteristic (ROC) curve was drawn to assess the values of PCTc and D-Dc on the clinical outcome of the severe pneumonia patients. SPSS statistics 22.0 was used for statistical analysis. Data comparison between two groups was perfomed using χ² test, t test or Fisher exact test depending on date type. Results Among the 78 elderly patients, 31 cases (39.7%) were transferred out of ICU after treatment, and the other 47 (60.2%) patients died. The survival group had significantly younger age [(77.30±6.72) years] and lower APACHE Ⅱ score [(19.29±2.69) points] than the death group [(83.84±5.54) years, (21.02±2.72) points; P<0.05]. There were no obvious differences in PCT level between the survival group and the death group before and in 3 d after treatment (P>0.05). But the level was statistically lower in the survival group than the death group in 7 d after treatment and before transference from ICU or death [(2.57±0.65) vs (7.18±1.91) ng/ml, (0.62±0.25) vs (10.37±2.47) ng/ml; P<0.01]. Before and in 3 d after treatment, no significant difference was seen in D-D level between the 2 groups (P>0.05). Its level was in a decreasing trend in the survival group but in an increasing trend in the death group, and the former group had obviously lower level than the latter group in 7 d after treatment and transference from ICU or before death [(0.75±0.15) vs (3.78±0.85) mg/ml, (0.40±0.09) vs (4.89±0.69) mg/ml, P<0.001]. Statistical difference was seen in PCTc level between the 2 groups in 3 and 7 d after treatment and before out of ICU or death [5.56 (3.28,8.22) vs -3.17 (-4.55,0.00),53.73(49.12,58.93) vs -31.75(-40.91,-21.05),89.29(86.36,91.78) vs -96.86(-119.40,-68.00),all P<0.001]. The D-Dc level maintained at a high level and gradually turned to elevation with the condition better in the survival group. But the level differed from the 2 groups in 3 and 7 d after treatment and before out of ICU or death [0.00 (0.00,6.25) vs 0.00(-7.69,0.00),58.33(55.29,64.21) vs -95.24(-128.50,-60.87),77.22(74.29,83.64) vs -161.10(-218.70,-114.20); all P<0.001]. ROC curve analysis showed that PCTc, D-Dc and APACHEⅡ were of prognostic significances for the elderly with severe pneumonia [area under curve (AUC)=0.974,0.779, 0.337; P<0.05]. PCTc had an AUC of 0.974, a 95% confidence interval of 0.940 to 1.010, cutoff value of 0.658, a sensitivity of 93.5%, and a specificity of 97.9%. Conclusion PCTc and D-Dc could predict the clinical prognosis of elderly patients with severe pneumonia, and show good value in clinical application.