Tryptophan?2,3?dioxygenase (TDO) is a heme?containing tetrameric protein that exhibits specific catalytic activity for L?tryptophan. TDO assists oxygen into the indole gene of tryptophan and then catalyzes it into kynurenine (KYN). Nervous tissues play an important role in the human body, but cannot regenerate. Tryptophan metabolism is closely related to the physiological function of nerves. Activation of TDO leads to the depletion of tryptophan and the accumulation of KYN and its metabolites, which can affect the function of neurons and cause related immune disorders. Mounting evidence obtained in recent studies indicates that TDO play an important role in the pathophysiology of several brain diseases. This paper summarizes the biological functions of TDO in the neurological diseases schizophrenia, Alzheimer’s disease, and glioma, with the aim of emphasizing TDO’s effect in tryptophan metabolic pathways and related neurological diseases. This should guide the development and application of TDO?specific inhibitors, which could provide new treatments for certain neurological diseases.