The invention discloses a compound capable of inhibiting IDO, its preparation method and its application. Its general structural formula is as follows: in, selected from a cis isomer, a trans isomer or a mixture of cis and trans isomers; R 1 and R 2 each independently selected from any one of hydrogen atom, halogen, alkyl, alkoxy or haloalkyl; R 3 Any one selected from cyclopentyl, cyclohexyl, piperazinyl, piperidinyl, R 3 The substitution positions are 1, 2, 1, 3 or 1, 4; m and n are integers of 0-5 respectively. The compound capable of inhibiting IDO provided by the present invention has strong inhibitory activity on IDO, and can be used to prepare IDO inhibitors to prevent and / or treat diseases with pathological characteristics of IDO-mediated tryptophan metabolic pathway, and has Very good application prospects.