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 ${\mathrm{MoS}}_2$ grown on GaN by depositing 3 nm of ${\mathrm{Al}}_2{\mathrm{O}}_3$ on the surface. The temperature-dependent Raman spectrum shows that the thermal stability of ${\mathrm{MoS}}_2$ is improved by ${\mathrm{Al}}_2{\mathrm{O}}_3$. Theoretical simulations confirmed the existence of tensile strain on ${\mathrm{MoS}}_2$ covered with ${\mathrm{Al}}_2{\mathrm{O}}_3$, and the bandgap and electron effective mass of six layers of ${\mathrm{MoS}}_2$ decreased due to tensile strain, which resulted in an increase of electron mobility. Due to the tensile strain effect, the photodetector with the ${\mathrm{Al}}_2{\mathrm{O}}_3$ 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 ${\mathrm{Al}}_2{\mathrm{O}}_3$. Our work provides an effective technical way for improving the performance of 2D material photodetectors.