Aiming to regulate crystallization and reduce defects of perovskite films, an organic small molecule with sulfonyl, 3-ethylbenzo[d]isothiazole 1,1-dioxide (PSAD), is introduced into perovskite precursor solution. A variety of characterization studies (nuclear magnetic resonance, Fourier transform infrared and X-ray photoelectron spectra) and density functional theory calculations are conducted to reveal the hydrogen bonding and coordination interactions between PSAD and perovskite. As a result, a perovskite film with large grains and reduced defects is obtained, along with the lessened non-radiation recombination, decreasing hole extraction energy loss, and ameliorative electron extraction. All these efficacies contribute to the enhanced efficiency and stability of the PSAD-treated devices. Our chemical strategy can help to explore the mechanism behind passivation to improve perovskite photovoltaics.