The invention relates to a novel indoleamine-2,3-
dioxygenase (IDO) inhibitor, as well as a preparation method and an application thereof. The inhibitor is compounds of a formula I or pharmaceutically-acceptable salt, solvate, polymorph, 
enantiomer or 
racemic mixture thereof, wherein R1 or R2 is independently selected from H, C1-C5 
alkyl, 
halogen and a 
substituent of a formula II and either of R1 and R2 is the 
substituent of the formula II; R3 is an 
aryl group; and n is equal to 0, 1, 2 or 3. Compared with the know IDO inhibitors, the novel IDO inhibitor has significant 
inhibitory effect on IDO and can be used for treatment of diseases having IDO-mediated 
tryptophan metabolic pathway pathological features, such as tumors, cancers, Alzheimer's 
disease, autoimmune diseases, cataract, psychological disorders, depression and / or 
anxiety. The preparation method of the novel IDO inhibitor has the advantages of being convenient in operation, having mild 
reaction conditions and reducing 
solvent consumption and 
pollution, thereby facilitating industrial production.