In order to investigate the characteristics of extrusion displacement in the tunnel face and advanced core in tunnel with weak surrounding rock, the extrusion displacement of the soft fault area F215 in the left line of Qishan tunnel in Fujian province is measured with the GMD sliding micrometers produced by Solexperts AG Company. A series of numerical simulations are carried out with finite difference program to study the characteristics of extrusion displacement during the time tunnel passes through the weak rock zone; and the influence of the length of weak rock zone and the stiffness ratio of hard rock to weak rock are analyzed. The results show that: (1) The magnitude of extrusion displacement can reflect the quality of the surrounding rock of the advanced core, while the distribution of extrusion displacement in advanced core can reflect the condition of joint crack ahead of the tunnel face. (2) The range of disturbance in front of the tunnel face caused by excavation are around 1.5 times as long as the excavation span of the tunnel. (3) The extrusion displacement is able to increase or decrease in advance before the tunnel face approaches the area where the surrounding rock is changed. (4) When the tunnel face enters into weak rock zone from hard rock zone, the length of weak rock zone can affect the magnitude and the change trend of extrusion displacement in some extent. (5) The bigger the stiffness ratio of hard rock to weak rock, the faster the rate of extrusion displacement change increases; and the earlier the extrusion change occurs when the tunnel face approaches from hard rock zone to weak rock zone, while there is no difference when the tunnel face approaches from weak rock zone to hard rock zone. (6) The monitoring and analysis of extrusion displacement can be a supplementary method of advance geological forecast to distinguish the condition of surrounding rock ahead of the tunnel face.