With fast growing demand for modern transportation, more shallow tunnels are being constructed and planned. Understanding the deformation and failure mechanism of a shallow underground tunnel has been a topic of research. The influences of the surrounding material (rock and soil) strengths and buried depths on the deformation and failure mechanism are investigated through the transparent soil model test technique and PFC3D numerical simulation in this study. It can be observed from model tests that the failure mode of test 1 (relative density of 30%, buried depth of 60 mm) is similar with that of test 2 (relative density of 70%, buried depth of 60 mm), both showing the funnel shape. The difference lies in that the tunnel stability in test 1 is lower. On the other hand, tests 2 and 3 (relative density of 70%, buried depth of 120 mm) are of different failure shapes as the latter displays a chimney shape. PFC3D simulation is carried out to numerically examine the failure modes under different testing conditions. The stress field and the displacement field derived from the numerical results can help to interpret the tunnel failure mechanism. In addition, it is obvious from this study that the Peck formula (1969) is less applicable for the surrounding materials with low strength. Therefore, it should be amended according to the surrounding materials, especially for the granular soils.