A　test　device　for　measuring　the　energy　density　of　the　focal　plane　of　the　trough　solar　collector　was　established　and　the　reflectivity　and　energy　flux　distribution　were　also　studied　in　this　paper　in　order　to　solve　the　reflectivity　reduction　and　energy　flux　loss　of　the　system　caused　by　dust　accumulation　of　the　mirror.　The　results　show　that　the　reflectivity　is　changed　in　different　mode,　in　which　the　reflectivity　of　the　long　wave　decreases　more　than　that　of　the　shorter　wave　in　the　visible　light　range　with　dust　accumulation　on　the　mirror.　The　sun　light　reflection　path　is　also　changed　due　to　dust　accumulation　of　the　mirror.　When　the　sun　direct　irradiance　increases　about　50　W/m2,　the　peak　of　the　energy　flux　in　the　wall　surface　of　the　receiving　pipe　is　reduced　about　15　kW/m2　under　the　condition　of　dust　accumulation　25　days　compared　with　that　of　the　clean　condition.　In　addition,the　convergence　light　path　is　changed　obviously　around　180°　of　the　circumferential　angle　of　the　receiving　tube.　The　functional　relationship　between　the　relative　reflectivity　and　the　dust　accumulation　period　and　density　was　obtained　according　to　the　data　measured,　which　can　also　be　used　to　predict　the　relative　reflectivity　of　the　trough　concentrator　in　adjacent　areas.　The　error　range　between　the　predicted　and　measured　value　is　around　±0.84.　This　work　provides　an　experimental　basis　for　determining　the　optimal　dust　removal　time.　©　2019,　Editorial　Board　of　Acta　Energiae　Solaris　Sinica.　All　right　reserved.