Provided in the present invention are a novel tricyclic aromatic heterocyclic compound and a preparation method therefor, a pharmaceutical composition and a use thereof. Specifically, provided in the present invention is a compound as shown in formula I below, or an optical isomer, a hydrate, a solvate thereof, or a pharmaceutically acceptable salt thereof. The definitions of each group are as specified in the description. The compound of formula I can be used for treating diseases related to a PD-1/PD-L1 signaling pathway.