The present invention relates to an imidazo isoindole derivative, a preparation method therefor and a medical use thereof. In particular, the present invention relates to the imidazo isoindole derivative as shown in the formula (I), a preparation method and pharmaceutical composition containing the derivative, and a use thereof for treating diseases with the pathological characteristic of IDO-mediated tryptophan metabolic pathways. The diseases comprise cancers, Alzheimer' s disease, autoimmune diseases, depression, anxiety disorders, cataracts, psychological disorders and AIDS, wherein the substituents in the formula (I) are the same as those defined in the description.