In this work, Bi2O3/Bi2SiO5 p-n heterojunction photocatalyst was successfully fabricated via a facile one-step synthesis using Bi(NO3)3 and nano-SiO2 as precursors. With the increasing amount of SiO2, α-Bi2O3 gradually transferred into β-Bi2O3 and Bi2O3/Bi2SiO5 p-n heterojunction was obtained at the same time. The as-prepared samples were systematically characterized by XRD, scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS). The Bi2O3/Bi2SiO5 heterojunction photocatalysts exhibited higher photocatalytic activity than α-Bi2O3 on the degradation of organic pollutants under simulated sunlight irradiation. The enhanced photocatalytic activity could be ascribed to the larger specific surface area, the larger contact angle, the formation of β-Bi2O3 and construction of p-n heterojunction. More importantly, the phase transition mechanism of Bi2O3 in Bi2O3/Bi2SiO5 heterojunction photocatalyst was proposed, which is significant for the theoretical study and application of photocatalytic materials.