In order to determine the round length reasonably in the process of tunnel excavation,the logarithmic spiral failure mode of advanced core soil on the tunnel face is proposed,which consists of two parts respectively considering vaults and advanced core soil in the surrounding rock.Then,in light of Janssen's silo theory and Mohr- Coulomb strength criterion,a calculation formula of vertical force loading on vaults is established,and the formula to calculate the round length in advanced core soil on the tunnel face,which is governed by such relevant parameters as internal friction angle,cohesive force,support pressure and excavation height,is set up on the basis of logarithmic spiral failure mode.The results show that ( 1) the round length produces very important impact on the stability of tunnel face,especially in the construction stage; ( 2) the support pressure on tunnel face considering round length may increase rapidly; and ( 3) the reinforcement of cohesive force and internal friction angle is of great significance for the improvement of round length and tunnel face stability,while the support pressure is of a critical value in terms of affecting tunnel face.