The influence of burial depth on the design of shallow tunnel is of great significance. In order to obtain the urrounding rock pressure distribution and influence length of shallow-buried section of tunnel portal under the condition of varying burial depths, the composition curve of the collapse body above the shallow buried section of tunnel is obtained based on Hoek Brown failure criterion and the upper limit theory of limit analysis, and the variation rule curves of the limit support force (the reaction force is surrounding rock pressure) along the axial direction of tunnel is obtained based on the variational principle. The analytical conclusions are drawn as follows: (1) The surrounding rock pressure increases in a curve with the increase of overburden thickness. (2) The effective influence range of the shallow-buried section can be obtained according to the relationship between the surrounding rock pressure and the distance (burial depth); and the surrounding rock pressure beyond the influence range hardly changes with the burial depth, which can be regarded as the deep-buried section. (3) The surrounding rock pressure and the critical range of the shallow-buried section are not only related to the geological conditions, but also related to the tunnel cross-section width and surface lope.