For matching lattice parameters, AlGaAs alloy is usually grown on a GaAs (001) substrate. The AlGaAs/GaAs multilayer structure has been widely used to manufacture various photoelectric and electronic devices. The practical importance of atomic flat surfaces lies in improving the performances of modern optoelectronic devices based on AlGaAs/GaAs multilayer structure. The influence of temperature on the flatness of the film has not been analyzed in detail, so it is very important to prepare the surface at an atomic level by adjusting annealing temperature. In this paper, 15 ML Al0.17Ga0.83As are deposited on an n-doped GaAs (001) substrate by the molecular beam epitaxy (MBE) technique. We study the effects of various annealing temperatures (520℃, 530℃, 540℃) on the flattening of Al0.17Ga0.83As/GaAs (001) surface under the same condition of arsenic BEP about 1.210-3 Pa, annealing time 60 min and growth rate (0.17 ML/s). The (1000 nm1000 nm) scanning tunneling microscope (STM) images and Fourier transform graphs are obtained to show the evolution of surface morphology. In a temperature range of 520-530℃, island is ripening, the coverage of the island increases, the pit also begins to merge into a larger pit; when the temperature exceeds 530℃, the increasing of ripening rate leads to a big island and the pit turns into terrace, while the coverage of island and the pit gradually decreases. In the annealing process, the area of terrace increases and gradually approaches to 100%. By quantitatively analysing the coverage of pit (island, terrace) and root mean square (RMS) roughness varying with the annealing temperature, a 545℃ (1℃) better annealing temperature is proposed by fitting the curve of RMS roughness variation. At the same time, the film annealing model is analyzed in this paper. Comparing the results in the literature with our experimental data, it is found that the change of annealing temperature can influence the number of active atoms, in which the ratio of annealing atoms contributing to surface flattening () should be proportional to the annealing temperature. According to the experimental results, Al0.17Ga0.83As surface basically presents the flat morphology with 60 min annealing at 540℃ when 0.20 0.25. When the annealing temperature reaches 545℃, we can also speculate that the annealing time is about 55-60 min. This is consistent with our previous conclusion. It should be pointed out that our experiment avoids metallizing the film surface caused by the anti-evaporation of the atoms and the metal gallium atoms climbing on the surface of the film when the annealing temperature is too high. The experimental results are applicable to the Al0.17 Ga0.83As thin film growth and annealing.