Abstract: To deeply understand the failure process and mechanism of spalling in deep straight-wall-top-arch tunnels, a true triaxial test is performed on the red sandstone cube sample (100 mm×100 mm×100 mm) with a straight-wall-top-arch hole using the TRW-3000 true triaxial test system to simulate the spalling process of the straight-wall-top-arch tunnel under the initial in-situ stress environment at depth of 500 m. The test process is monitored and recorded by using a video surveillance system. The failure process and failure characteristics of the sidewalls during the tests are analyzed, and compared with the failure of the circular hole sidewall at the same depth. The results show that under the condition that the vertical stress is the maximum principal stress and the horizontal radial stress is the minimum principal stress, the failure of straight-wall-top-arch tunnel mainly occurs in the area between the hance and the spandrel. The rock near the free surface is fractured into plate-like thin rock slabs that are approximately parallel to the maximum principal stress, which is characterized by typical tensile spalling fracture. With the increase of the maximum principal stress, spalling gradually develops toward the horizontal radial direction of the hole, and eventually forms a symmetrical V-shaped damage zone. The spalling slabs exhibit the arc-shaped feature with thick middle and thin wings, and spalling has obvious time effect. Compared with those of the circular hole sidewall, the dynamic failure characteristics of the straight-wall-top-arch hole sidewall are more prone to static failure, and the initial failure requires higher stress, but the sidewalls are more severely damaged under a high stress environment.