Numerical simulation is an effective way to analyze blast response of underground protective arches. The efficiency of the numerical method is weakened when a three-dimensional (3D) structure is simplified as a two-dimensional (2D) structure. A comparative study of blast response of buried arches with 2D and 3D models was carried out. In 2D model, restraints to the arch are strengthened by the model and responses, including strains, stresses and vibration velocities, should be under-estimated. It is found that the computer results for the free field and structure responses by 2D and 3D model could agree well just on the condition when the blast scaled distance beyond a range. To the buried arches, the critical scaled distance is 2.76 m/kg1/3.