Objective To investigate the correlation of serum level of 25-hydroxy vitamin D3 (25(OH)D3) with the severity of heart failure, and evaluate its clinical value. Methods A total of 152 patients who met the diagnostic criteria of 2014 Chinese Heart Failure Diagnosis and Treatment Guidelines were recruited in this study. They were 78 males and 74 females, at an age of (72.7±12.7)years, and according to the results of the New York Heart Association (NYHA) Functional Classification, they were assigned into NYHA-Ⅰ group (group A, n=44), NYHA-Ⅱ group (group B, n=51) and NYHA-Ⅲ-Ⅳ group (group C, n=57). After serum 25(OH)D3 level and other biochemical indices were detected, the differences in the 3 groups and the correlations of the serum 25(OH)D3 level with the severity of chronic heart failure were analyzed. All the data were analyzed by SPSS statistics 19.0, and Student’s t test, analysis of variance, least significant difference (LSD) test and Chi-square test were adopted for intergroup comparison. Linear regression analysis was employed to analyze the correlation of the 25(OH)D3 level with heart failure. Results There were significant differences in brain natriuretic peptide (BNP), left ventricular ejection fraction (LVEF) and hemoglobin (Hb) in the 3 groups (P<0.05). The 25(OH)D3 level was (22.53±8.17), (18.13±6.69) and (14.55±6.09)ng/ml respectively in the groups A, B and C, and significant difference was seen among them (P<0.05). The results of LSD test showed that the level of 25(OH)D3 was significantly higher in group A than in group B and group C (P<0.05). The linear regression correlation analysis indicated that the level was negatively correlated with BNP and age (P=0.000, P=0.002), but positively with LVEF (P=0.000). Conclusion 25(OH)D3 level is negatively correlated with the severity of chronic heart failure. But whether vitamin D supplementation is beneficial to alleviating symptoms and improving prognosis needs further large-scale study.