Disclosed are a new derivative as shown by general formula (I' ) or a pharmaceutically acceptable salt thereof, a pharmaceutical composition containing the derivative or the pharmaceutically acceptable salt thereof, and a method for preparing the derivative or the pharmaceutically acceptable salt. Disclosed is the use of the derivative or the pharmaceutically acceptable salt thereof or the pharmaceutical composition containing the derivative or the pharmaceutically acceptable salt thereof in the preparation of a medicine, in particular an IDO inhibitor medicine and a medicine for treating and/or preventing a tumour. Each substituent of the general formula (I' ) is the same as defined in the description.