We　used　the　traditional　hydro-thermal　synthesis　method　to　generate　the　first　photochromic　pyrimidinium/polyoxometalate　(POM)　salt　material　(C4N2H4–C4N2H3)　[Mn(H2O)6]0.5{SiW12O40}0.5ˑ5H2O　in　situ　by　using　silicotungstic　acid　and　pyrimidine　as　raw　materials.　The　solid　UV　diffuse　reflectance　spectrum　of　compound　1　was　shifted　before　and　after　illumination.　There　was　a　new　broad　absorption　peak　appeared　near　370　nm　and　624　nm　after　illumination.　With　the　increase　of　irradiation　time,　the　peak　at　624　nm　will　shift　to　588　nm　at　which　the　sample　reached　light　saturation.　The　color　of　the　sample　changed　from　yellow　to　grayish　green.　By　studying　a　series　of　test:　EPR,　XPS,　UV–Vis,　X-ray　single　crystal　diffraction,　and　density　of　states　(PDOS)　calculation,　we　conclude　that　the　charge　transfer　mechanism　of　photochromism　may　be　due　to:　the　generation　of　pyrimidinium　free　radicals　by　light-induced,　photochemical　reduction　of　polyoxometalates　and　the　interaction　of　two　aspects.　This　work　enriches　the　family　of　photochromism　and　its　theory.　©　2020　Elsevier　B.V.