Solar　light　pipe　combined　with　Photocatalysis　to　depredate　formaldehyde　under　cloudy　conditions　in　summer　were　tested　in　Beijing.　The　solar　light　pipe　used　in　the　experiment　is　a　straight　solar　light　pipe,　which　is　550mm　long,　with　diameters　of　280mmm.　Degussa　P25　titanium　dioxide　(＂Degussa＂,　Germany)　as　photocatalyst　was　coated　on　the　surface　of　the　emitter　of　the　solar　light　pipe.　The　test　chamber　made　up　of　stainless　steel　is　0.1m(3).　The　experiment　results　showed　obvious　effect　on　decomposing　formaldehyde　of　solar　light　pipe　combined　with　Photocatalytic　under　cloudy　conditions.　The　volume　fraction　of　formaldehyde　reduced　from　1.0x10(-6)　to　0.16x10(-6)　during　35min　and　the　value　reduced　from　1.0x10(-6)　to　0.10x10(-6)　during　60min　under　cloudy　conditions.　The　experiment　showed　that　Photocatalytic　by　the　radiations　of　sunlight　under　sunny　conditions　also　has　good　effect　for　air　cleaning.