Cesium-based inorganic metal-halide perovskites have attracted a great deal of scientific attention as effective emissive materials in light emission, due to their high luminescence quantum yield and outstanding thermal stability. However, if deposited with a one-step low-temperature solution method, these perovskite films always suffer from poor film quality and low crystallinity, leading to high trap state density and low photoluminescence quantum yield (PLQY). Herein, we prepared cesium-based perovskite films with a polymeric interlayer assisted crystallization and interfacial passivation method, taking advantage of forming an adduct [polyvinyl pyrrolidone (PVP)-Pb2+]. Compared with the film prepared with the traditional method, the trap state density in the modified perovskite films was significantly suppressed. The stability was also improved. Therefore, the PLQY of the perovskite film deposited with the one-step low-temperature method was largely enhanced. The corresponding light emitting diodes based on the modified films exhibit an approximately 4-fold increase in current efficiency compared with the control device. These findings reveal that interfacial engineering is an efficient pathway to fabricate high quality cesium-based perovskite films with the one-step solution method.