In order to study the process of crack propagation under uniaxial stress，the crack propagation process and the stress intensity factor of the front edge of the crack are obtained by the numerical simulation of the threedimensional crack calculation software Franc 3d.The results show that：（1）under uniaxial tension，the wing crack first appears at the tip of the precast crack，and then the crack propagates around the specimen.The crack first runs through both sides of the specimen and then through the front and back of the specimen.（2）in the case of three dimensional precast internal crack inclination α=0 degree and 30 degree，the two wing cracks at the tip of the precast crack tip exhibit mutual attraction during the later stage of crack growth.When the angle of internal crack is 45°and 60°，there is no tendency to attract and approach each other between wing cracks，and the propagation of wing crack at the inner tip of precast crack is smaller than that on the outer tip of precast crack.（3）the relative mode Istress intensity factor changes with the distance of prefabricated crack tip（from relative distance 0 to relative distance 1）.Compared with the modeⅡ stress intensity factor，the stress intensity factor decreases first and then decreases and then increases with the change of the distance between the prefabricated crack tip and the crack tip.