Objective To investigate the changes in interleukin-34 (IL-34)levels in serum and bronchoalveolar lavage fluid (BALF) of patients with severe pneumonia and their prognostic value. Methods A total of 66 patients with severe pneumonia (severe pneumonia group), 35 patients with non-severe pneumonia (non-severe pneumonia group), and 27 healthy adults (control group) were enrolled. The severe pneumonia group was further divided into survival group of 38 patients and non-survival group of 28 patients based on 28-day survival. Clinical data of all subjects were analyzed. Receiver operating characteristic (ROC) curves were plotted to assess the predictive power of serum IL-34 and relative IL-34 gene expression in BALF for 28-day mortality in patients with severe pneumonia, as well as the predictive power of serum IL-34 for severe pneumonia. Kaplan-Meier survival curves were plotted, and the Log-rank test was used to compare cumulative survival rates. Cox regression analysis was conducted to explore risk factors for 28-day mortality in patients with severe pneumonia. Pearson correlation analysis was used to assess the correlation between serum IL-34 levels and relative IL-34 gene expression in BALF of patients with severe pneumonia. Results Serum IL-34 levels were higher in the severe pneumonia group than those in the non-severe pneumonia group, and were higher in the non-severe pneumonia group than in the control group (P < 0.05). Serum IL-34 levels and relative IL-34 gene expression in BALF were higher in the non-survival group than in the survival group (P < 0.05). ROC curve analysis showed that the areas under the curve for predicting 28-day mortality in patients with severe pneumonia were 0.908 for serum IL-34 levels and 0.878 for relative IL-34 gene expression in BALF. The optimal cutoff value for serum IL-34 levels in predicting severe pneumonia was 129.9 pg/mL. Multivariate Cox regression analysis showed that increased serum IL-34 levels and increased relative IL-34 gene expression in BALF were independent risk factors for 28-day mortality in patients with severe pneumonia (P < 0.05). The Kaplan-Meier survival analysis results indicate that, with a cutoff value of 129.9 pg/mL, patients with severe pneumonia who had high serum levels of IL-34 exhibited a lower cumulative survival rate compared to those with low IL-34 level(Log-rank P < 0.001). Conclusion Serum IL-34 levels are significantly increased in patients with severe pneumonia, and relative IL-34 gene expression in BALF is positively correlated with serum IL-34 levels. Both can be used as indicators for predicting the prognosis of 28-day mortality in patients with severe pneumonia.