Abstract: Under the control of different strain amplitudes,cyclic softening and hardening characteristic of high-strength low-alloy 800MPa grade 10CrNi5MoV steel at axial loading was studied.The experimental results manifested that the steel shows different cyclic softening and hardening characteristic by loading at different strain amplitudes.When stain amplitude changes from 0.30% to 1.0%,the steel indicates cyclic softening feature and the strain amplitude is higher,the cyclic softening rate is quicker and when strain amplitude changes from 0.30% to 0.40%,the material indicates a specialty of hardening,cycle saturation,slightly softening,fracture.Dislocation state of specimens at different strain amplitudes was observed under TEM and the above cyclic softening and hardening phenomena was analyzed by dislocation theory.