The effect of growth pressure on the properties of β-Ga2O3 thin films epitaxial on c-plane sapphire substrate with 6° oblique cut angle by plasma-assisted molecular beam epitaxy were systematically studied. All of the epitaxial films present (201) orientated single-crystalline structure with smooth surface morphology. In addition, the crystalline quality and growth rate increase gradually with the increase of the growth pressure, as demonstrated by X-ray diffraction and scanning electron microscopy characterizations. According to the X-ray photoelectron spectroscopy measurement results, the percentage of oxygen vacancy and Ga3+ oxidation states decrease and increase, respectively, with increasing growth pressure, resulting in the increment of atomic ratio of O to Ga. Moreover, through the calculation of Tauc frormula and Urbach tail model, the results show that the optical band gap of the films increase from 4.94 eV to 5.00 eV, while Urbach energy decreases from 0.47 eV to 0.32 eV. These results suggest that the crystal quality and optical property of β-Ga2O3 were improved by increasing the growth pressure.