The　magnetic　field　effect　on　heterogeneous　photocatalytic　degradation　of　benzene　over　Pt/TiO2　has　been　observed　by　on-line　GC.　Time-resolved　photoconductivity　and　in-situ　FTIR　were　used　to　investigate　the　influence　of　magnetic　field　on　the　photogenerated　charge　carrier　lifetime　and　the　intermediate　distribution,　respectively.　The　results　showed　that　in　the　presence　of　magnetic　field　the　effective　lifetime　of　the　photogenerated　charge　carriers　was　prolonged　with　the　increase　of　the　magnetic　field　intensity.　The　results　also　indicated　that　the　application　of　the　external　magnetic　field　not　only　favored　the　conversion　of　benzene,　but　also　facilitated　the　transformation　from　phenol　to　quinone,　resulting　in　improving　the　activities　of　the　photocatalytic　degradation.