Compounds as IDO inhibitors and/or dual inhibitors of IDO-HDAC are described. Specifically, the compounds represented by the following formula (I) are described, wherein each group is defined as described in the specification. The compounds have IDO inhibitory activity or IDO-HDAC dual inhibitory activity and can be used for preventing or treating diseases associated with IDO and/or IDO-HDAC activity or expression levels. At the same time, the compounds of the present invention can be combined with an anti-tumor antibody such as PD-1 and PD-L1, and such a combination can greatly increase the anti-tumor response rate of the antibody and broaden the types of tumors to be treated.