Orthorhombic　Bi2SeO5　with　a　flower-like　microsphere　is　synthesized　by　a　phase　transformation　synthesis　process　for　the　first　time.　This　new　photocatalyst　has　a　bandgap　energy　of　3.5　eV　and　its　fermi　level　locates　at　-0.86　V　vs　NHE　from　the　results　of　UV-vis　DRS　and　Mott-Schottky　plot.　The　effects　of　the　calcination　temperature　and　reaction　time　on　the　morphology,　photoabsorption　performance,　photoelectric　property　and　photocatalytic　activity　have　been　investigated　in　detail.　The　as-prepared　Bi2SeO5　exhibits　highly　efficient　photocatalytic　activity　in　the　decomposition　of　organic　dyes　and　aromatic　compound,　such　as　rhodamine　B,　methyl　orange　and　bisphenol　A　in　aqueous　solution　under　the　simulative　sunlight　irradiation.　The　high　photocatalytic　activity　of　Bi2SeO5　may　be　attributed　to　the　high　crystallinity,　the　hierarchical　structure　and　the　high　separation　and　mobility　of　the　charge　carrier.　Notably,　the　effect　of　the　surface　area　on　the　activity　could　be　neglected.　During　the　photocatalytic　reaction　process,　Bi2SeO5　shows　very　high　stabilities　among　the　structure,　surface　chemical　state　and　activity.　Furthermore,　the　DMPO　spin-trapping　ESR　spectra　and　the　control　experiments　reveal　that　the　center　dot　O-2-radicals　and　photoinduced　holes　play　an　important　role　in　the　photocatalytic　degradation　process.　Finally,　a　probable　photocatalytic　mechanism　of　Bi2SeO5　has　been　proposed.