Abstract: Ground-water depletion and preliminary fault are the main factors arousing the ground fissures hazard in Xi'an City.Firstly,the tensile crack machanisim,shear crack machanisim,shear and tensile multiple machanisim of the ground fissure are analysis in theory,and the controling role of the preliminary fault on the causes of ground fissure is analysed based on structrual plane controling theory; Then,authors set up a numerical simulation model with three dimension biot consolidation theory,through this model,the displacement of three cases,pumping without preliminary fault,pumping with preliminary fault,pumping without preliminary fault but with stratum diffrence,are compared,the authors found that the preliminary fault is not only the tectonic foundation,but also has induction,amplification and segregation fuction on the formation and development of the ground fissues.