Organometal halide perovskites have recently gained remarkable interest as a light absorber for photovoltaic technology. However, the majority of efficient perovskite solar cells (PSCs) are fabricated in the glovebox. From the mass production perspective, it is vital to improve the quality of ambient-processed perovskite, as well as to reduce health risks, costs, and complexity of fabrication. This study investigates the effect of using green antisolvent ethyl acetate with various halide additives on the performance of ambient-processed planar PSCs by a one-step deposition method, under a high relative humidity (RH) of 50–60%. Differences in microstructures, crystallinity, surface chemistry and optoelectronic properties for the green antisolvent and ambient-processed perovskite films with the addition of PbCl2, PbBr2, and MAI are observed, compared to the pristine film. Ambient-processed perovskite films with PbCl2 addition remarkably improves the PCE of the pristine device by 17.3% and achieves a fill factor up to 76.3%. Addition of PbCl2 also enhances the stability of the devices and retains 81% of their initial PCEs after storage in ambient air for 28 days. These findings provide a strategy of using halide additives and green antisolvent for the fabrication of high-quality perovskite films under high relative humidity without the need of glovebox.