Provided in the present application are a sulfonyl amidine as represented by formula (I) as an indoleamine-2,3-dioxygenase inhibitor, and a preparation method therefor and the use thereof. The compound of formula (I) in the present application can be used as an indoleamine-2,3-dioxygenase inhibitor in the preparation of a drug for preventing and/or treating indoleamine-2,3-dioxygenase-mediated diseases.