Abstract: The low cycle fatigue damage process of S321 and S321H austenitic stainless steel was studied, and the influence of carbon element content on their fatigue life was analyzed. The results show that at room temperature, the low cycle fatigue process of S321 and S321H steels went through four stages, initially mild cyclic hardening, less obvious saturation, rapid cyclic hardening, and final fracture. The materials also went through a cyclic softening stage at high temperature. As the mass fraction of carbon element increased, the content of deformation martensite in the material decreased and the fatigue life extended. As the strain amplitude increased, the content of deformation martensite in the material increased and the fatigue life shortened. S321H steel exhibited defects such as secondary cracks, second phase particles, and pores at higher strain amplitudes, resulted in a shortened fatigue life of the material.