To better utilize the infrared (IR) region in sunlight for photovoltaic devices (PVs),upconversion nanoparticles (UCNPs) have been proposed to improve power conversion efficiency (PCE).However,researchers recently have found that the upconversion (UC) effect is negligible in PVs performance improvement for their ultra-low UC photoluminescence quantum yields of UCNPs solid film,while the real mechanism of UCNPs in PVs has not been clearly studied.Herein,based on the material inorganic perovskites γ-CsPbI3,NaYF4:20％Yb3+,2％Er3+ UCNPs were integrated into different transport layer to opti-mize device performance.Compared with reference device,the short-circuit current density and PCE of optimized device reached 20.87 mA/cm2 (20.39 mA/cm2) and 18,34％ (17.72％),respectively,without sac-rificing open-circuit voltage and filling factor.Further experimental characterizations verified that the improved performance was attributable to enhanced visible light absorption instead of IR.To theoreti-cally explain the statement,the light field distribution in device was simulated and the absorption in dif-ferent layers was calculated.The results revealed that the introduction of UCNPs with different refractive index from other layers caused light field disturbance,and improved visible light captured by γ-CsPbI3.Importantly,through experiments and theoretical calculation,the research deeply explored the potential mechanism of UCNPs in optimizing PVs performance.