The present invention relates to fused heterocyclic derivatives, processes for their preparation and their use in medicine. Specifically, the present invention relates to a novel derivative represented by the formula (I' ), or its pharmaceutically acceptable salt thereof, a pharmaceutical composition containing the derivative or its pharmaceutically acceptable salt thereof, and the method for preparing the derivative and its pharmaceutically acceptable salt thereof. The present invention also relates to the use of the derivative and its pharmaceutically acceptable salt thereof, or a pharmaceutical composition containing the derivative and its pharmaceutically acceptable salt thereof in the preparation of medicines, in particularly as IDO inhibitor medicines, for treating and/or preventing cancers. Wherein each substituent of the formula (I' ) is the same as defined in the specification.