Abstract: Objective To investigate the relationship between homocysteine (Hcy) and cognitive impairment in patients with Parkinson's disease (PD). Methods Total 228 patients with PD admitted to our hospital were enrolled in the study, who were divided into cognitive normal group (78 cases) and mild cognitive impairment group (76 cases) and dementia group (74 cases) according to the results of relevant diagnostic criteria and screening methods, and 70 healthy people from the physical examination center of our hospital as the control group. The general information (gender, age, education level), Hoehn-Yahr grading, duration of illness, other risk factors (hypertension, diabetes, hyperlipidemia), dopasine and pramipexole hydrochloride taking of all patients were collected. Plasma Hcy levels, serum folate and Vit B12 levels were measured, and the Montreal Cognitive Assessment (MoCA) scores were evaluated. Baseline data, plasma Hcy levels, and MoCA scores were compared among the four groups. Results Plasma Hcy levels in the control group, PD cognitive normal group, PD mild cognitive impairment group and PD dementia group increased in turn, and the MoCA scores decreased in turn (P < 0.05). Pearson correlation analysis showed that plasma Hcy level was negatively correlated with MoCA score of PD patients (r=-0.684, P=0.017). Conclusion Plasma Hcy levels were negatively correlated with cognitive impairment in PD patients. The higher the plasma Hcy level, the lower the MoCA score, and the more severe the cognitive dysfunction in PD patients.