Objective To investigate the prognostic value of prognostic nutritional index (PNI) for prostate cancer patients. Method A total of 124 prostate cancer patients who were treated in Hai'an Hospital Affiliated to Nantong University from May 2016 to November 2019 were included in this study. All patients were divided into high PNI group (> 50.2, n = 62) and low PNI group (≤50.2, n = 62) based on the median value of PNI. The correlation between PNI and clinicopathological features of prostate cancer patients was analyzed, and Kaplan⁃Meier curves were plotted to evaluate the impact of pretreatment PNI on the overall survival of these patients. In addition, the Cox multivariate regression analysis was performed to determine the independent prognostic factors for prostate cancer patients. Result Low PNI was significantly correlated with TNM stage (P=0.031), but not with patient age, smoking history, prostate specific antigen (PSA) level, Gleason score and treatment methods. In this cohort, the median OS of prostate cancer patients was 43.0 months (range: 27.6-58.4). Kaplan⁃Meier survival curve showed that the OS of patients with low PNI was significantly shorter than that of those with high PNI. The 3⁃year OS of two patient groups was 50.9% and 79.9% respectively, with a significantly statistical difference(χ 2=10.967, P=0.001). Multivariate Cox regression analysis revealed that PNI before treatment(HR=3.174, 95%CI=1.122-8.978, P=0.029), Gleason score (HR=2.888, 95%CI=1.094-7.625, P=0.032) and TNM stage (HR=3.179, 95%CI=1.116-9.058, P=0.032) were independent prognostic factors for pretreatment prostate cancer patients. Conclusion PNI is an independent prognostic factor for pretreatment prostate cancer patients, and it might be used as an useful indicator of prognostic assessment.