Abstract: Stability of tunnel surrounding rock, a core issue in underground tunnel design, has been a hot research field in tunnel engineering, and an accurate assessment of the stability is an important precondition for construction safety under complicated geological conditions. Aiming at this issue, first we conduct laboratory tests of tunnel surrounding rock to examine its behaviors of gradual destruction and reveal its staged development characteristics of instability and failure. Then, through analysis of the instability mechanism, a mechanical model of tunnel surrounding rock instability and a method for calculating the instability thresholds are developed. According to its stable behaviors and instability modes, tunnel surrounding rock is divided into inner and outer layers, and thus a compound surrounding rock structure model can be constructed. This study reveals that its developing stages of instability are essentially a consequence of the outer surrounding rock moving in a mode of grouping, and the characteristics of its structure stability are thus analyzed. For application to complicated and variable ground conditions, a systematic study is carried out focusing on the structure of composite surrounding rock in different stratigraphic conditions, and it shows the "composite arch" effect of the sand surrounding rock and the "beamed arch" structure and "multi-layer arch" effect of layered surrounding rock, together with the stability characteristics of these types of surrounding rock. Finally, a new stability control technology of complicated tunnel surrounding rock is presented in this paper. This study lays a theoretical basis for safety of tunnel construction and design of tunnel support.