The crack propagation of three-dimensional internal crack under tensile load has been a difficult and hot problem in geotechnical engineering. In order to quantitatively explore the crack propagation and the change of stress intensity factor of standard cubic specimen with three-dimensional internal crack under uniaxial tensile load, the variation of the crack propagation law of different prefabricated internal crack angles under tensile load was simulated numerically, and the crack propagation process and the final failure morphology were compared with the existing research. The results showed that the precast cracks exhibit self-similar propagation when the precast crack angle is 0 degree and when the precast cracks are at other angles, the wing crack propagates at the tip of the precast crack. When the angle of the precast crack is 15 degrees, 30 degrees, 45 degrees, 60 degrees, the wing crack and the original precast crack present an angle of 18 degrees, 49 degrees, 64 degrees and 80 degrees. With the increase of the angle between the precast crack and the horizontal direction, the relative stress intensity factor of mode I decreases gradually, and for the same prefabricated crack angle, the relative mode Ⅰ stress intensity factor on the short axis vertex of prefabricated crack is larger than that in other directions. For the mode Ⅱ stress intensity factor, the relative mode Ⅱ stress intensity factor increases first and then decreases with the increase of the prefabricated crack angle. For the same precast crack angle, the left and right end of the precast crack (corresponding to the distance of 0/0.5/1) reaches the maximum for the mode II stress intensity factor, while the relative II type stress intensity factor of the vertex of the short axis of the precast crack (corresponding to the distance of 0/0.5/1) reaches the minimum of 0 at 0.25,0.75, and the numerical simulation results are in agreement with the laboratory test results. The research results provide a certain reference for the correct understanding of the three-dimensional crack propagation law under the condition of axial tension.