Abstract: Excavation dimension is an important index for progress and economy in building underground excavation project. How to determine a reasonable, safe, efficient, rapid, economic and optimal excavation dimension mainly depends on whether the advanced support measures can prevent the vault surrounding rock from falling or collapsing in period of excavation completion to initial support. Taking a 14.6 m span tunnel in Qingdao stratigraphic tectonic zone as the engineering example, this paper studied and analyzed the variation law of pipe shed deflection with excavation dimension from five aspects, namely pipe diameter, pipe wall thickness, circumferential spacing, load height and elastic resistance coefficient. The conclusions were as follows: ①The optimal excavation dimension under the different diameter pipe shed support increased as the pipe diameter increasing, when the optimal excavation dimension corresponding to different pipe diameters was the same, priority should be given to small diameter pipe shed.②The optimal excavation dimension was not sensitive to the wall thickness of pipe shed. It was recommended that 8~12 mm medium wall pipe shed should be selected first. ③The ring distance of the pipe shed of 0.35~0.45 m was reasonable, which could realize optimal excavation dimension construction and give full play to the role of pipe shed in preventing local block falling.④The optimal excavation dimension between 1.5~3.0m gradually increased with the decreasing of the calculated load height, it was recommended to take reliable measures to reduce the relaxation and dispersion of surrounding rock in order to further expand the optimal excavation dimension. ⑤A threshold value of initial support elastic resistance coefficient of 7MPa/m was found, and it was essential to find a reasonable initial support stiffness and strictly control the construction quality to ensure that the elastic resistance coefficient was not lower than the threshold value.