Parkinson's disease (PD) is one of the most common neurodegenerative diseases in the elderly, which is mainly characterized by dyskinesia and is often accompanied by non-motor symptoms (NMS) such as autonomic nervous dysfunction, anosmia, sleep disorders, psychiatric symptoms (depression and anxiety) and cognitive dysfunction. Psychiatric symptoms and cognitive dysfunction are the most common NMS. So far, the mechanisms involved in the pathophysiology of the NMS have been unclear. The serotonin (5-hydroxytryptamine, 5-HT) transmitter system contributes to the pathophysiology of various brain functions and neuropsychiatric disorders, and is closely related to the NMS of PD. This article reviews the research progress in the role of the 5-HT transmitter system in psychiatric symptoms and cognitive impairment associated with PD, expounds the distribution and function of the 5-HT transmitter system in the brain, and illustrates the changes of the 5-HT transmitter system under PD, attempting to provide research data for the understanding and treatment of PD-related psychiatric symptoms and cognitive impairment.