Flexible　perovskite　solar　cells　(PSCs)　have　attracted　tremendous　attention　due　to　their　potential　application　in　portable　and　wearable　electronics.　However,　the　photoelectric　conversion　efficiency　(PCE)　of　flexible　PSCs　is　still　far　lower　than　that　of　usual　rigid　PSCs.　Moreover,　the　mechanical　stability　of　flexible　PSCs　cannot　meet　the　needs　of　commercial　applications　because　of　the　cracking　of　perovskite　grains　caused　by　bending　stress.　Here,　we　introduced　a　spacer　cation　additive　(2-(chloromethyl)　pyridine　hydrochloride,　CPHC)　within　the　perovskite　organic　precursor　to　improve　the　device　PCE　and　its　mechanical　stability.　We　observed　that　the　CPHC　spacer　cation　additive　could　simultaneously　facilitate　the　crystallization　of　perovskite　and　stitch　the　grain　boundaries　to　improve　the　flexibility.　Compared　to　the　17.64%　PCE　of　the　control　devices,　the　target　flexible　PSCs　achieved　a　more　highly　efficiency　over　19%　with　an　improved　mechanical　stability　(87.2%　of　the　initial　PCE　after　the　1000　cycles　with　the　bending　radius　R　=　6　mm).　In　addition,　compared　to　methylammonium　or　formamidinium　cation,　due　to　the　stronger　hydrophobic　and　larger　activation　energy　barrier　for　the　ion　migration　of　the　CPHC　spacer　cation,　the　device　retained　over　80%　of　the　initial　PCE　after　30　days　storage　in　an　ambient　environment.