Grain boundaries (GBs) have a significant role in polycrystalline perovskite solar cells (PSCs). However, there is ongoing debate regarding the impact of GBs on the performance and long-term stability of PSCs. Employing the first-principles molecular dynamics for perovskites, the iodine vacancy defect migrations both in bulk and at GBs are investigated. i) The positive iodine vacancy (VI+) is found that have both lower formation energy (1.4 eV) and activation energy (0.18 eV) than those of neutral iodine vacancy (VI), statistically. It indicated the VI+ acts as the dominant migrated iodine vacancy rather than VI; ii) the iodine vacancy at GBs has approximate to 0.48 eV higher activation energy than those in bulk, which leads to the accumulation of iodine vacancy at GBs; iii) the presence of VI+ result in a 3-fold increase in charge recombination ratio at GBs, compared to pristine PSCs. Based on quantum molecular dynamics statistical results, which are consistent with experimental measurements, insights into iodine vacancy migration both at GBs and in the bulk are gained. This understanding can be valuable for defects engineering related to ion migration, in order to improve the long-term stability and promote the performance of PSCs. Understanding defects engineering related to ion migration is crucial for enhancing the long-term stability and performance of hybrid perovskite solar cells. Iodine vacancies accumulate at grain boundaries due to lower formation energy and higher migration potential barrier compared to those in the bulk, which further increase the nonradiative recombination. image