The optical properties of ZnO thin-film ridge-waveguide are studied, and the reduction of scattering loss inside the random cavities can also be obtained by covered ZnO films with MgO capped layer. Additionally, the finite difference time domain(FDTD) method for numerical calculation is utilized. The simulation results show that random laser with low scattering loss and directional optical output from the facets can be reached by employing annealed ZnO thin-film ridge-waveguide with a MgO capped layer.