Iodine vacancies and uncoordinated iodide ions of CsPbI3 films are mainly responsible for nonradiative recombination. Here, we report a composition-engineering passivation method that through guanidium (GA+) and I− forms strong hydrogen bonds to passivate iodine vacancies and reduce defects. Both experimental and theoretical results confirmed strong chemical interactions between GA+ and uncoordinated I− in the GAxCs1−xPbI3 bulk or at the grain boundary. Moreover, GA+ doping could slow down the crystallization speed of perovskite films during the deposition process. As a result, we observed GA+ modified films with much lower defect density, larger grain size, and better carrier extraction and transportation. Upon GA+ passivation, the power conversion efficiency (PCE) is boosted from 18.01% to 19.05%, with open-circuit voltage (VOC) enhancement from 1.08 V to 1.14 V.@en