A　high-efficiency　of　25.9%　for　perovskite/Si　heterojunction　tandem　solar　cells　is　achieved　with　an　open-circuit　voltage　of　1.74　V　contributed　by　approximate　to　1.02　and　approximate　to　0.72　V　from　the　top　and　bottom　cells,　respectively.　The　results　demonstrate　that　the　addition　of　methylammonium　chloride　(MACl)　effectively　mitigates　the　open-circuit　voltage　loss　in　the　perovskite　cell,　where　the　MA　cation　tunes　the　bandgap　while　the　Cl　anion　facilitates　grain　growth　with　less　boundaries.　Meanwhile,　the　excess　lead　iodide　(PbI2)　caused　by　the　evaporation　of　MACl　plays　a　beneficial　passivation　role　to　achieve　high　open-circuit　voltage.　The　performance　of　the　tandem　solar　cells　is　a　synergic　consequence　of　all　cations　and　anions　considering　the　thickness,　bandgap,　phase　stability,　and　defect　in　the　perovskite　absorber.