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• Deformation of 18 sections in Grade V rock mass and the initial support structural force of 2 sections are analyzed. • FLAC 3D numerical simulation is used to study the deformation of the rock mass. • The supporting parameters of fifth-grade surrounding rock for large-span loess highway tunnel are designed. When a large span loess tunnel is excavated, the rock mass is disturbed, and it is difficult to form a load-bearing arch under shallow depth region. As a result, the structural deformation is complicated. This paper analyzed the deformation of 18 sections in Grade V rock mass and the primary support structural force of 2 sections in a case study of a highway loess tunnel in the large span loess highway tunnel group of Huangyan Traffic Capacity Expansion Engineering in Northern Shaanxi Province-Jianzicha No.2 Tunnel. The analyzed results were combined with the FLAC 3D numerical simulation to study the deformation of rock mass in Jianzicha No.2 tunnel and the stress characteristics of primary support. The results showed that: (1) The time-space relationship curve of arch deformation indicated three stages of "rapid deformation stage -continuous deformation stage-slow deformation stage" in the construction by annular excavation with reserved core soil method. (2) The rock mass pressure distribution of tunnel basically presented the bilateral symmetry characteristics. The rock mass pressure distribution of arch was relatively uniform, and the rock mass pressure at the invert was relatively large. (3) The outer and inner sides of primary support arch steel rib was subjected to compression, and the stress distribution was relatively uniform and large. (4) The primary support shotcrete was mainly subjected to compression, and the stress distribution was basically characterized by bilateral symmetry. The stress distribution of shotcrete at the arch was relatively uniform and large, and the stress of shotcrete at the inverted part was relatively small. This paper studied the time-space law of supporting structure deformation in the shallow section of Ⅴ grade surrounding rock, and designs the supporting parameters of Ⅴ grade surrounding rock for large-span loess highway tunnel, which has guiding significance for the construction of similar projects in the future. [ABSTRACT FROM AUTHOR]