Many fatigue cracks can be derived from tank under long-term working conditions. These cracks not only affect the structural strength and service life of tank, but also threaten the personal safety of operators. Therefore, the analysis and research on the dynamic expansion of fatigue cracks in tank materials and the life mechanism are carried out. In this paper, semi-elliptical crack specimens with different length-to-depth ratios were prefabricated by Franc3D crack analysis software to study the change of dynamic stress intensity factor of cracks under fatigue tensile loading, and to investigate the crack expansion path, rate and life time. The results show that the distribution law of the dynamic stress intensity factor at the tip of the semi-elliptic cracks with different length-depth ratios is basically the same, and the magnitude of the intensity factor at the near surface is the highest. The final crack morphology is still semi-oval in final. At the beginning of crack growth, crack propagation rate is almost equal, the growth distance increases exponentially with the increase of the number of load cycles, and the propagation rate at the near surface is the highest, but the crack propagation rate at the near surface of the crack with length-depth ratio c/a less than or equal to 1 is basically the same.