Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their excellent optoelectronic properties and low preparation cost. However, a large number of defects exist on the surface and grain boundaries of perovskite films, which lead to non-radiative recombination of carriers and affect the photoelectric conversion efficiency of solar cells. In this work, acetylsalicylic acid (ASA) was introduced into the lead salt solution as a passivator to prepare perovskite films and solar cells via a two-step method. The impacts of acetylsalicylic acid on film quality and cell performance were investigated by means of optical spectroscopy, scanning electron microscopy and various electrical measurements. The results show that the introduction of appropriate amount of ASA can obviously increase the grain size of perovskite films and effectively reduce the defect density of perovskite films via Lewis acid-base interaction. The solar cell prepared with the incorporation of 2.5 mmol/L ASA achieves the highest photoelectric conversion efficiency of 19.83%, appreciably outperforming the control device (17.47%). This work highlights the efficacy of ASA in passivating defects of perovskite films and provides a facile method to improve the performance of perovskite solar cells.