The present invention discloses a five-membered heteroaryl ring derivative (I), and an isomer, a solvate, a crystalline form of the solvate, a prodrug, a stable isotope derivative, or a pharmaceutically acceptable salt thereof. The five-membered heteroaryl ring derivative of the present invention has good IDO inhibitory effect, and can effectively treat, alleviate, and/or prevent various related diseases caused by immunosuppression, such as tumors, infectious diseases, and autoimmune diseases.