The demand for large underground space is increasing as various needs arise. As the scale of underground space increases, failure and collapse risks accordingly rise, primarily due to higher uncertainty of properties and a lower arching effect. Therefore, proper primary support systems (e.g., rock bolt and shotcrete) are required to prevent failure of the tunnel face during excavation and large vertical displacement at the tunnel crown and ground surface for safe tunnel construction. Additionally, a pre-improvement support system must be applied before excavation and during the excavation process, depending on the tunnel scale. The necessity of a pre-improvement support system for large underground excavation is verified based on numerical analyses. Important design parameters of the pre-improvement support system are chosen and their effectiveness in terms of tunnel support is analyzed through parametric studies according to various tunnel scales. The results suggest that the reinforcement area around the tunnel is not linearly dependent on the tunnel scale and the required reinforced area for a large tunnel is wider than that of the linearly obtained reinforced area as the tunnel scale increases. Finally, this paper presents the optimized design parameters of a pre-improvement support system according to tunnel scale and also presents more general evidence of the need for a pre-improvement support system in large underground spaces.