All-inorganic perovskite materials, typically CsPbI2Br, have received widespread attention owing to their outstanding thermal stability than that of organic/inorganic hybrid counterparts. However, the power conversion efficiency (PCE) of CsPbI2Br perovskite solar cell (PSC) is far lower than its theoretical value. In this study, to further improve the PCE, a sulfur-rich small molecule material (delta-2:2-bis (1,3-dithiazole)), is used to modify the interface between CsPbI2Br and carbon electrode. Encouragingly, the carbon-based CsPbI2Br PSCs achieve a high PCE of 13.78% than the control of 10.40%, which is the best performance of carbon-based CsPbI2Br PSC among the literature report at present. The remarkable reduction of defect density and suppression recombination should be responsible for the PCE improvement. This work proposes a simple and effective strategy to enhance the efficiency of all-inorganic carbon-based CsPbI2Br PSCs.