Abstract: This paper deals with the rules of hydrogen absorption of three types of HAZ structure formed in welding 30CrMnSiNi2A ultra-high strength steel,including the related diffusion coefficient and solubility of hydrogen in the overheated region of HAZ,the changes on the behavior of hydrogen diffusion after subzero treatment,and the sensitivity of the structues to hydrogen embrittlement.The authors investigated the influence of the simulated structure of the HAZ on the behavier of hydrogen diffusion by using the methods of transmission electron microscope,cathodic electrolysis and definite load delaycd fracture test.It has been noted that the improvment in structure of the overheated region is the essential condition for the improvoment in proporty of the post-heated HAZ.The filmlike retained austenite spread over between the martensite lath packets plays an important role in raising the proporty of HAZ.