The　power　conversion　efficiency　of　organic-inorganic　hybrid　perovskite　solar　cells　has　increased　rapidly,　but　the　device　stability　remains　a　big　challenge.　Previous　studies　show　the　grain　boundary　(GB)　can　facilitate　ion　migration　and　initiate　device　degradation.　Herein,　methimazole　(MMI)　is　employed　for　the　first　time　to　construct　a　surface　patch　by　in　situ　converting　residual　PbI2　at　GBs.　The　resultant　MMI-PbI2　complex　can　effectively　suppress　ion　migration　and　inhibit　diffusion　of　the　metal　electrodes.　The　origin　of　the　surface　patch　effect　and　their　working　mechanisms　are　investigated　experimentally　and　theoretically　at　the　microscopic　level.　It　hence　demonstrates　a　simple　and　effective　method　to　prolong　the　device　stability　in　the　context　of　GB　engineering,　which　could　be　extensively　applied　to　perovskite-based　optoelectronics.