Abstract: Considering the influences of unsupported length during the process of tunnel excavation, a complete set of centrifugal test device is developed combined with the manipulator system. Based on the centrifugal tests on C/D and internal friction angle under different unsupported lengths, the variation laws of support force and the mechanism of instability mode of different soil excavation faces under the consideration of unsupported length are studied, and the double-block instability body above the unsupported length should be considered. The results show that considering the unsupported condition, the instability of soil is composed of the soil above the unsupported part and the soil in front of the excavation face, and the two parts do not lose stability at the same time, which breaks the traditional understanding of the instability of the excavation face. With the increase of C/D, the instability of soil gradually changes from two parts to one part, and the support force also decreases. The length of unsupported part increases linearly with the support force of excavation face, and shortens the time for soil to enter the instability stage. With the increase of the internal friction angle, the ultimate support force gradually decreases and tends to a stable value, but has no effects on the instability mode.