Abstract: The microscopic photos in different locations of annealed low carbon steel welded joint were obtained after the welded joint was cathodic electrolyte hydrogen-charged to produce hydrogen damage. The diffusion coefficient, average hydrogen concentration and diffusion hydrogen content in deferent locations of welded joint were calculated and the hydrogen permeation curves were measured with electrochemical method, in order to explain the reason why different locations of the welded joint had different numbers of hydrogen bubbles, hydrogen blistering and hydrogen induced cracking after hydrogen charging. It is shown that the diffusion coefficient of base metal was far less than that of weld beam, the average hydrogen concentration of base metal was far larger than that of weld beam, leading to that hydrogen permeation and hydrogen damage behavior of base metal was more obvious. The hydrogen bubbles from surface spillover and hydrogen blistering produced near the surface of base metal were far more than those of weld beam. However, the plasticity and toughness of weld beam was lower than those of base metal, and the residual tensile stress in weld beam was higher than that in base metal, thus more hydrogen induced cracks formed in weld beam mostly near the surface because of the higher hydrogen concentration, and a few elongated cracks generated inside weld beam due to larger restraint.