Reversible　magnesium　(Mg)　plating/stripping　is　a　key　challenge　for　Mg　metal　batteries.　Hence,　a　dual-functional　additive,　tin(II)　fluoride　(SnF2),　is　introduced　into　the　simple　Mg　salts　(Mg(OTf)2　and　Mg(TFSI)2)　electrolytes　with　a　pure　amine　solvent　(3-methoxypropylamine,　MOPA).　Interfacial　characterizations　and　theoretical　calculations　reveal　that　the　dissociated　SnF2　can　drive　the　in-situ　formation　of　a　multiphase　Sn-based　solid-electrolyte　interphase　(SEI)　on　the　Mg　anode.　Such　a　robust　and　magnesiophilic　SEI　shields　the　Mg　metal　from　MOPA-induced　corrosion　and　parasitic　reaction,　while　facilitates　the　interfacial　kinetics　of　Mg2+.　Concurrently,　the　competitive　coordination　of　F-　anions　from　SnF2　raises　the　contact　ion　pairs　and　further　mitigates　the　MOPA　decomposition.　As　a　result,　the　reversibility　of　Mg　metal　anode　is　successfully　enhanced,　and　Mg//Cu　cells　maintain　over　1000　cycles　at　2.0　mA　cm-2　with　an　average　Coulombic　efficiency　exceeding　99.8%.　This　work　offering　a　promising　pathway　toward　practical　application　of　high-performance　Mg　metal　batteries.