Abstract：As porous medium，rocks are affectd by ground water permeation. Elasto-plastic theory and damage mechanics are used to obtain the stress distribution of rocks around tunnel and the relation of porous hydraulic pressure P0 and plastic radius r. According to the theory of limit point of stability，the stable condition for rocks around tunnel is put forward，and the values of critical porous hydraulic pressure of rocks around tunnel are acquired by taking the ground water permeation effect into account. When the porous hydraulic pressure value of rocks around tunnel is close to the critical value，the rocks around tunnel are in the state of instability. If dynamic disturbance is present under this condition，the rocks around tunnel will collapse due to instability and the accident of water irruption will take place.
