The emergence of organic-inorganic hybrid perovskites with a high μτ product and a high absorption coefficient has made it possible to adopt an aerosol-liquid-solid technology for direct X-ray detectors. The film quality from the ALS process is often compromised, especially on the film surface, when deposited in ambient conditions with uncontrolled humidity. Herein we develop a solvent engineering strategy in the ALS process to obtain high-quality MAPbI3 thick films. The key is the introduction of a molecular additive to intervene and regulate the perovskite crystallization process so that the negative effect of the ALS ambience is minimized. This strategy allows us to prepare direct X-ray detectors with much reduced dark current, enhanced response speed and improved overall performance.