Because　the　advantages　of　non-direct　contact,　remote　control,　high　efficiency　and　fastness,　light-driven　reversible　color　switching　materials　(LDRCSMs)　have　important　applications　in　information　storage,　display　devices,　sensors,　and　other　fields.　The　combination　of　inorganic　materials　and　organic　materials　to　achieve　synergies　is　one　of　the　research　hotspots　of　new　LDRCSMs.　A　one-step　liquid-phase　synthesis　method　was　used　to　prepare　Mg-doped　TiO2　nanocrystals　(Mg-TiO2　NCs)　with　a　particle　size　of　similar　to　5　nm.　The　reversible　light-responsive　color　switching　of　methylene　blue　(MB)　by　using　Mg-TiO2　NCs　was　investigated.　The　results　show　that　Mg2+　doping　generates　impurity　levels　in　the　TiO2　lattice,　effectively　inhibiting　the　recombination　of　photogenerated　carriers　and　increasing　the　activity　of　TiO2　photooxidation-reduced　MB.　On　the　other　hand,　Mg2+　doping　can　decrease　the　absorption　wavelength　of　TiO2　NCs,　which　effectively　represses　the　partial　reduction　of　MB　to　reduced　LMB　under　visible　light　irradiation,　leading　to　a　higher　recoloration　rate.　Such　LDRCSMs　based　on　alkali　metal-doped　TiO2　NCs　may　have　potential　application　in　many　optoelectronic　device　fields.