While all-inorganic halide perovskites (iHPs) are promising photovoltaic materials, the associated water sensitivity of iHPs calls for stringent humidity control to reach satisfactory photovoltaic efficiencies. Herein, we report a moisture-insensitive perovskite formation route under ambient air for CsPbI₂Br-based iHPs via cesium cyclopropane acids (C₃) as a compound introducer. With this approach, appreciably enhanced crystallization quality and moisture tolerance of CsPbI₂Br are attained. The improvements are attributed to the modified evaporation enthalpy of the volatile side product of DMA-acid initiated by Cs-acids. As such, the water-involving reaction is directed toward the DMA-acids, leaving the target CsPbI₂Br perovskites insensitive to ambient humidity. We highlight that by controlling the C₃ concentration, the dependence of power conversion efficiency (PCE) in CsPbI₂Br devices on the humidity level during perovskite film formation becomes favorably weakened, with the PCEs remaining relatively high (>15 %) associated with improved device stability for RH levels changed from 25 % to 65 %. The champion solar cells yield an impressive PCE exceeding 17 %, showing small degradations (<10 %) for 2000 hours of shell storage and 300 hours of 85/85 (temperature/humidity) tests. The demonstrated C₃-based strategy provides an enabler for improving the long-sought moisture-stability of iHPs toward high photovoltaic device performance.