Thermostability and band gap deeply influence the photocatalytic activity of the photocatalysts. In this article, BiOBr and La-doped BiOBr photocatalysts were synthesized via a solvothermal process. The obtained samples were calcinated at different temperatures. X-ray diffraction (XRD), N-2-physical adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetry analysis (TGA) and differential thermal analysis (DTA) were utilized to characterize the effect on thermostability after La doping. UV-Visible spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, Electrochemical Impedance Spectroscopy (EIS), transient photocurrent spectroscopy and Mott-Schottky (M-S) plots were utilized to analyze the band gap and band edge positions of the samples. The results presented that La doping not only enhanced thermostability of BiOBr but also regulated the band gap of heterojunction to advantageous status for promoting the activity. These results could open a new way to tune the heterojunction with rare earth doping by heat treatment.