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 GA_xCs_1−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 (V_OC) enhancement from 1.08 V to 1.14 V.