So far, it’s been widely acknowledged that the PbI2 decomposition under illumination mainly accounts for the degradation of perovskite solar cells (PSCs) under maximum power point (MPP) tracking condition. However, PSCs without excess PbI2 were also reported to deteriorate rapidly under the same condition. Here, we demonstrate that the key to enhance PSCs stability under MPP tracking condition is not to have fascinating surface morphology with effective suppression of nonradiative recombination traps but to prevent the migration of iodine ion (I−) under light illumination. By partially substituting methylammonium chloride (MACl) with methylammonium iodide (MAI) and simutaneouly introducing I2 during the sequential deposition, the iodine vacancies in perovskite films are substantially suppressed, thereby limiting the pathways for I− migration. As a consequence, PSCs with efficiency of 24.28% are fabricated with remarkably enhanced working stability.