Objective To investigate the correlation between brain-derived neurotrophic factor（ BDNF） level in serum and Parkinson disease depression（ PDD）. Methods A total of 56 patients with Parkinson disease（ PD） and 56 healthy controls were recruited. Among the 56 PD patients， there were 23 cases of PDD and 33 cases of non-PDD. All patients' serum BDNF level was examined. Depression was assessed by PHQ-9 for PD patients. All the PD patients were evaluated with Patient Health Questionnaire-9（ PHQ-9） for their depression degree， and Hoehn-Yahr grading scale for PD patients' motor function. Results The serum BDNF level of patients with PD was（ 7.28±1.49） ng/ml， which was significantly lower than that of the control group（ 11.87±2.03） ng/ml， and the difference was statistically significant（ t=2.35，P＜0.01）. The serum BDNF level of patients with PDD was（ 4.95±1.33） ng/ml， which was lower than that of the patients with non-PDD（ 9.60±1.67） ng/ml， and the difference was statistically significant（ t=3.45，P＜ 0.01）. The comparison of serum BDNF level of PDD patients with different depression levels showed that the serum BDNF level of the severe PDD group was the lowest. The serum BDNF levels of all PDD groups were compared and analyzed with hierarchical correlation（ rs=-0.24， P ＜ 0.01）. Conclusions Serum BDNF level decreased in PD patients. Serum BDNF level was negatively correlated with PD depression. The decrease of BDNF level may be involved in the pathophysiology of PDD.