Non-radiative recombination of perovskite solar cells (PSCs) will increase as a result of the numerous crystallographic defects that the solution-grown perovskite films will cause, particularly at the grain boundary and film surface. As a result, it negatively influences the performance of the device. Consequently, lowering perovskite film defects is a useful strategy for raising the efficiency of PSCs. This study reports a grain regeneration and passivation approach that can decrease the recombination loss of the perovskite layer/charge transfer layer interface and the grain border. Guanidine iodide (GAI) treatment of perovskite films is the means by which this objective is accomplished. Unlike most methods that use GAI to post-treatment the perovskite layer or add GAI into the perovskite precursor solution, this work uses GAI for pre-treatment before spin coating the perovskite layer. It can effectively passivate surface defects and increase the grain size of perovskite films by controlling the crystallization process. The water stability of devices was enhanced, the short-circuit current (J_sc), filling factor (FF), and power conversion efficiency (PCE) of PSCs were markedly improved, and non-radiative recombination was successfully reduced. The best efficiency of PSCs was 20.56% after the additional GAI treatment was applied to the perovskite layer, an 11.9% increase over the efficiency of the control device without GAI treatment. This method has the advantage of being simple and straightforward, providing a feasible pathway for the low-cost preparation and commercialization of PSCs.