Some large earth fissures always come into being wing to fault movement and pumping groundwater aggravates their activity.The detail mechanism is rather complicated and is seldom examined.Theory of failure and slide of the weak plane is used to study the detailed mechanism of the reactivation and development of earth fissures during pumping in situation of pre-existing faults.The results indicate that the deformation and failure mode of soil mass are both changed greatly during pumping if there is a weak plane.The stress disturbance aroused by water table decline is the main factor causing the rock/soil body to slide along the weak plane.This result is different from previous view points that differential settlement is caused by stratigraphic difference or subsidence funnel is the main control factor.The research further completes the forming and development mechanism under pumping.