The　even　and　transparent　nanometer　TiO2　thin　films　named　DegussaP25　as　photocatalysis　deposited　on　the　surface　of　diffusers　of　solar　light　pipe　were　prepared　by　sol-gel　processing.　The　rugged　side　of　the　diffusers　of　solar　light　pipe　was　coated　evenly　with　DegussaP25　solution　for　the　quality　of　1.75　g.　The　experiments　had　showed　that　when　the　coated　side　was　away　from　the　sun　the　lighting　degree　may　be　reduced　compared　with　that　facing　the　sun.　The　average　reduction　was　only　3.03%,　which　would　not　have　a　significant　impact　on　lighting.　Diffusers　are　important　parts　of　a　light　pipe　which　can　diffuse　light　evenly　to　the　place　needed　to　be　illuminated.　The　experiments　showed　that　in　a　sunny　summer　day　under　the　direct　sunlight,　the　solar　light　pipe　combined　with　photocatalysis　could　reduce　the　formaldehyde　volume　fraction　in　a　box　of　0.1　m(3)　from　1.0x10(-6)　to　0.16x10(-6).　After　1　h　of　photodegradation　the　formaldehyde　volume　changed　from　1.0x10(-6)　down　to　0.1x10(-6)　with　faster　and　more　complete　degradation　of　formaldehyde.　The　rate　of　degradation　under　cloudy　and　partly　cloudy　conditions　was　slower　than　that　under　sunny　conditions.　It　was　slower　in　winter　than　in　summer　under　sunny　conditions.　The　experimental　results　also　showed　that　the　performance　of　photocatalysis　combined　with　diffusers　of　light　pipe　had　better　effect　in　a　small　space.　The　performance　in　large　space,　such　as　open　space,　will　be　the　next　work　in　the　future,　which　will　be　a　great　challenge.