Although antibody-based immune checkpoint blockades have been successfully used in antitumor immunotherapy, the low response rate is currently the main problem. In this work, a small-molecule programmed cell death-ligand (PD-L1) inhibitor, LG-12, was developed and radiolabeled with 131 I to obtain the chemically and biologically identical radiopharmaceutical [ 131 I]LG-12, which aimed to improve the antitumor effect by combination of LG-12 and [ 131 I]LG-12. LG-12 showed high inhibitory activity to PD-1/PD-L1 interaction. The results of cell uptake and biodistribution studies indicated that [ 131 I]LG-12 could specifically bind to PD-L1 in B16-F10 tumors. It could induce immunogenic cell death and the release of high mobility group box 1 and calreticulin. The combination of [ 131 I]LG-12 and LG-12 could significantly inhibit tumor growth and resulted in enhanced antitumor immune response. This PD-L1 small-molecule inhibitor based combination strategy has great potential for tumor treatment.