Hybrid organic-inorganic perovskites have recently gained tremendous attention as they have enabled bright, widely tunable light-emitting diodes, photodetectors with high detectivity, as well as solar cells that have reached certified power conversion efficiency (PCE) to 22.1% for single junction devices. The detection of ultraviolet (UV) light has a wide range of applications, such as chemical, environmental and biological analysis and monitoring, flame and radiation detection, astronomical studies, and optical communications. UV-A in the range of 320 to 400 nm can penetrate deep into the skin and cause skin damage. Hence, UV-A photodetectors are highly desired to monitor the UV-A intensity. Methylammonium lead chloride (CH3NH3PbCl3) as a wide-bandgap perovskite (Eg = 3.17 eV) shows a high optical absorption coefficient in UV-A range while transparent to visible light, which makes it a promising candidate for visible-blind UV-A photodetectors. In this work, CH3NH3PbCl3 perovskite thin films were fabricated via a two-step spin coating and solvent-vapor-assisted thermal annealing method under low temperature. A PbCl2 DMSO solution with the concentration of 350 mg/mL was spin coated onto cleaned ITO-coated glass substrates followed by thermal annealing at 70 °C for 10 min. A CH3NH3Cl 2-propanol solution with the concentration of 20 mg/mL was then spin coated onto the PbCl2 layers and annealed at 70 °C for 30 min to facilitate interdiffusion. DMSO-vapor-assisted thermal annealing was introduced at 100 °C after the interdiffusion annealing for 1 h. The films exhibited cubic crystalline structure verified with XRD and pinhole-free morphologies from SEM images. The possible charge traps were investigated via the analysis of photoluminescence spectra of perovskite films prepared with different lead chloride (PbCl2) precursor concentrations while maintaining the same concentration of methylammonium chloride (CH3NH3Cl). The PL results showed that the CH3NH3PbCl3 perovskite thin films fabricated with 350 mg/mL PbCl2 and 20 mg/mL CH3NH3Cl exhibited the PL peak at the shortest wavelength and almost no tail following the PL peak, indicating that the film had less trap states than films made with other PbCl2 concentrations. Prototypical UV photodetectors with the structure of ITO/CH3NH3PbCl3/Poly (triaryl amine) (PTAA)/Al were fabricated and showed low dark current density 1.60 × 10-5 mA/cm2 under -1 V reverse bias, strong photoresponse in 300-400 nm region, and a high UV-visible rejection ratio (defined by responsivity in UV range divided by that at 500 nm) up to 500 under 0 or -0.5 V bias. All the results demonstrated that low-temperature solution-processed CH3NH3PbCl3 perovskite thin films offer a great potential for making flexible, lightweight visible-blind UV-A photodetectors.