Abstract: Low cycle fatigue behavior and fracture mechanism of 45 CrNiMoVA steel under in-situ electrochemical hydrogen charging current densities of 0 m A/cm², 2 m A/cm² and 4 m A/cm² were conducted. The results show that45 CrNiMoVA steel exhibits unsaturated cyclic softening closely related to strain amplitude and non-Masing characteristics during cyclic loading, and the non-Masing characteristics are more significant at small strain amplitudes. Although the hydrogen charging current density has negligible effect on the cyclic hysteresis behavior of materials, but the fatigue resistance of materials is significantly dependent on the magnitude of H-charging current density and strain amplitude. Specifically, the higher the current density, the higher the hydrogen concentration inside the material, and the more severe the fatigue damage caused to the material. Notably, hydrogen induced fatigue life degradation is more pronounced under high-strain amplitude conditions compared to the low-strain amplitudes. Scanning electron microscopy（SEM） fracture analysis shows that the hydrogen charging obviously changes the fatigue fracture mechanism: the specimen without hydrogen charging exhibits a typical surface crack initiation and ductile fracture characteristics; as the hydrogen charging current density and strain amplitude increase, the crack initiation position of the hydrogen charged specimen shifts from the surface to the internal defect position, and the brittle characteristics of quasi-cleavage and intergranular separation morphology become more obvious. The crack initiation zone and propagation zone of the hydrogen charged specimen exhibit mixed fracture characteristics with the coexistence of dimples, quasi-cleavage and intergranular separation morphologies.