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.