The formation and aggravation of hydrogen damage in Fe-3wt-%Si alloy during cathodic charging were studied by means of Lange transmission X-ray topography. Results showed that hydrogen damage did not form in the specimens charged in 0.5 mol/L H_2SO_4 solution without poison, and occurred with addtion of 250 mg / L As_2O_3 even at very low current density. As charging at a certain current density, the size of the damage may enlarge up to a limit yet no more by prolonged time. The damage size increases with increase of charging current density, but not so apparent at higher current density. An external tensile stress could promote obviously the formation of hydrogen damage. It seems that for charging in H_2SO_4 solution containing poison, the drop of hydrogen permeation curve against time prolongation is due to the formation of hydrogen damage.