The present invention provides a polycyclic compound that acts as an IDO inhibitor and/or an IDO-HDAC dual inhibitor. Specifically, the present invention provides a compound represented by the following formula (I), the definition of each group being as described in description. The compound has IDO inhibitory activities or IDO-HDAC dual inhibitory activities, and can be used for preventing or treating diseases associated with IDO and/or IDO-HDAC activity or expression level. In addition, the compound of the present invention can be administered in combination with anti-tumor immune antibodies such as PD-1 and PD-L1, so as to greatly improve the anti-tumor response rate of the antibody and broaden the range of treatable tumor types. (I)