In recent years, MoSe₂, as a kind of transition metal dichalcogenide, has aroused widespread research interest due to its special crystal structure with different electrical and optical properties. The band gap of molybdenum diselenide can be manipulated by different layers, strain engineering, doping, or the formation of heterostructures, which makes it potential advantages in optoelectronic devices and photovoltaic applications. In this work, we investigate the influence of selenization temperature on the structures and optical properties of the MoSe₂ films. Molybdenum (Mo) thin films are prepared by RF magnetron sputtering, and then MoSe₂ thin films are generated by selenization annealing. The surface morphology, crystal structure, and optical bandgap for each of the MoSe₂ thin films are characterized and analyzed by using scanning electron microscopy, X-ray diffraction, and ultraviolet visible spectroscopy, respectively. The results show that the crystal structures of the MoSe₂ thin films are closely related to the selenization temperature (T_s): with the increase of selenization temperature, the average grain size in the thin film decreases slightly and then increases rapidly from 24.82 nm to 55.76 nm. Meanwhile, the (002) crystal plane of MoSe₂ also exhibits preferential growth with temperature increasing. Each MoSe₂ thin film has a low absorption rate for short-wavelength light (around 600 nm). With the increase of selenization temperature, the bandgap waves of the MoSe₂ thin films are blue-shifted, and the optical bandgaps decrease, which is attributed to the fact that different selenization temperatures cause the lattice size of MoSe₂ to change, thereby affecting the spatial expansion of its electronic wave function. In addition, the structure and optical bandgap of MoSe₂ can be effectively controlled by changing the selenization temperature, which provides more possibilities for the applications of the MoSe₂ thin films in optical devices.