Abstract: Conventionally,the traditional design of shotcrete support structure has assumed that support structure is a prismatic layer with uniform thickness. Due to the influence of blasting,the surrounding rock surface of the tunnel with drilling and blasting method has different degrees of over-under-excavation,which results in the uneven thickness of the initial support structure layer. In the consideration of the possibility of rock surface irregularities caused by over- under-excavation,this paper,according to the shotcrete thickness,divides the irregular surface of the tunnel after sup- porting into three categories:not fully filled,partially filled,and fully filled. With utilizing the thick-wall cylinder theory of elastic mechanics,the theoretical formula of the parameters,such as equivalent support stiffness and maximum sup- port pressure,are established under the support of shotcrete or shotcrete +steel grid. Based on the research findings mentioned above,this work analyzes the stability of a tunnel excavation project with the aid of the convergence-con- finement method and calculates the safety factor of the support structure. The results show that the proposed method has a preferable applicability to the stability study of over-under-excavated tunnel,and the availability is verified pre- liminarily.