Strain regulation as an effective way to enhance the photoelectric properties of two-dimensional (2D) transition metal dichalcogenides has been widely employed to improve the performance of photovoltaic devices. In this work, tensile strain was introduced in multilayer MoS2 grown on GaN by depositing 3 nm of Al2O3 on the surface. The temperature-dependent Raman spectrum shows that the thermal stability of MoS2 is improved by Al2O3. Theoretical simulations confirmed the existence of tensile strain on MoS2 covered with Al2O3, and the bandgap and electron effective mass of six layers of MoS2 decreased due to tensile strain, which resulted in an increase of electron mobility. Due to the tensile strain effect, the photodetector with the Al2O3 stress liner achieved better performance under the illumination of 365 nm wavelength, including a higher responsivity of 24.6 A/W, photoconductive gain of 520, and external quantum efficiency of 8381%, which are more than twice the corresponding values of photodetectors without Al2O3. Our work provides an effective technical way for improving the performance of 2D material photodetectors.