Abstract: The influence of welding heat input,preheat temperature and the deposited metal diffusible hydrogen content were considered,the HAZ critical stress of X80 pipeline steel was tested by implant test based on the orthogonal regression design.The significant factors on the critical stress were obtained by analysis of variance.By multiple linear regression,the critical stress equation and the cold cracking susceptibility criterion were established.The critical stress equation was analyzed.It was concluded that the initial diffusing hydrogen content had great influence on cold cracking susceptibility of X80 pipeline steel.When the initial diffusing hydrogen content is lower,the microstructure is found to be the most important factor on the critical stress.When the initial diffusing hydrogen content is higher,the residual hydrogen content has great influence on the critical stress as well as the microstructure.