In recent years, perovskite materials become a research hotspot in the field of solar cells due to their excellent photovoltaic properties, but control of their interface defects still remains one of the key problems to be solved. In this study, an organic small molecule additive (L-3-(4-pyridyl)-alanine (PLA)) was introduced in the preparation of perovskite photoabsorption layer by two-step solution method. The characterizations showed that the introduction of PLA could comprehensively improve photoelectric performance of the device, with optimal energy conversion efficiency of 21.53%, in contrast to that of the reference device (20.10%). Further studies showed that PLA could reduce the trap state density of the device from 5.59×10¹⁶cm^-3 to 3.40×10¹⁶cm^-3, promote the interface charge extraction, and decrease the carrier recombination. The above improvements can be attributed to the PLA induced PbI₂ enrichment at grain boundaries and PLA anchoring at defects, which play an important roles in passivate defects. This study can provide guideline for further regulating the defects of perovskite solar cells.