It is generally known that Zn volatilization would result in a serious degradation in film quality and device performance during a non-stoichiometric ZnGa2O4 target ablation. This work highlights the effect of various substrate temperatures and partial oxygen pressures on the microstructural and optoelectronic properties of stoichiometric ZnGa2O4 films. First-principle studies have been performed to calculate the formation energy per atom of pure Ga2O3, ZnGa2O4, and ZnO to understand the mechanism of preparing ZnGa2O4 by pulsed laser deposition, revealing the stability of Ga2O3 > ZnGa2O4 > ZnO under various growth parameters. Sufficient oxygen partial pressure should be introduced to deposit a large amount of ZnO, and then the Ga:Zn ratio can be adjusted to the most suitable 2:1 by controlling the substrate temperature and post-annealing heat treatment. The X-ray diffraction analysis revealed the high crystalline nature of stoichiometric ZnGa2O4 film. The average transmittance of above 80% and wide energy gap of 5.18 eV revealed the potential of stoichiometric ZnGa2O4 film in deep-ultraviolet photodetectors. This stoichiometric ZnGa2O4 film exhibited the photo/dark current ratio of 3.63 × 104 and high responsivity of 1.97 A/W at the bias of 5 V under the illumination wavelength of 220 nm for deep-ultraviolet applications.