Abstract: The law of microdistributions for diffusible hydrogen in welded zone for four kinds of HSLA steels under different pretreatment is studied with the method of measuring the hydrogen bubbles under the microscope by video-recorder. According to the analysis of both the hardnees and the microstructure it has been found that the distribution of diffusible hydrogen in welded zone is consistent with variant sine-law ,and there is local accumulation of hydrogen in fusion zone and the juncture between coarse grained and finegrained zone.The results show that the nonuniform distribution of hydrogen trap is main influence factor of local accumulation of diffusible hydrogen. The law mentioned above,provides a theoretical basis for analying the influenees of the diffusible hydrogen on the welding cold cracking and preventing the welding cold cracking in engineering practice.