Nonequilibrium molecular dynamics (NEMD) simulations are investigated to obtain an understanding of the effect of strains on the thermal conductivity of Ge thin films. The results show that the strains have an appreciable influence on the thermal conductivity of Ge thin films. The thermal conductivity decreases as the tensile strain increases and increases as the compressive strain increases. It is explained by the fact that the decrease of the phonons velocities and the surface reconstructions. Meanwhile, a theoretical calculation basing on Modified-Callaway model under different strains is performed to assess our NEMD simulation results. We find that the theoretical results closely match the molecular dynamics results. The theoretical analysis reveals that the contributions of the relaxation time on strains are very important to the thermal conductivity of Ge thin films.