According to the typical shallow buried large section tunnel in the Yan Kou Mountain, considering the increase of the thickness of the concrete under the complicated geological conditions, the tunnel deformation is controlled to achieve the purpose of stabilizing the tunnel. Based on FLAC3D simulation of the three step core soil construction method, this paper analyzes the effect of different thickness of the concrete shotcrete layer on the stability of the tunnel construction process, to determine the reasonable support parameters. Through the modeling of the early support different spray layer thickness numerical simulation, surrounding rock plastic area and spray concrete bending moment diagram analysis, the results show that: concrete spray layer thickness change can reduce the deformation of the tunnel, which has little effect on the force of the tunnel structure; the thickness of the shotcrete layer is recommended to select the 25~30cm.%针对雁口山典型的浅埋大断面隧道，在高原复杂的地质条件下，考虑增大混凝土的厚度控制洞室变形，达到稳定隧道的目的。基于FLAC3D模拟隧道三台阶核心土施工法，分析不同厚度的混凝土喷层对隧道施工过程中稳定性的作用，确定合理的支护参数。通过建模对初期支护不同喷层厚度的数值模拟、围岩的塑性区及喷混凝土弯矩图的分析，表明：混凝土喷层厚度的改变可减少隧道变形，对于隧道结构受力改变甚微；建议初支混凝土喷层厚度选取25~30cm。
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