Abstract: The residual diffusive hydrogen H_R100 in the welded zone of HSLA (high strength low alloy) steels while cooled down to 100℃ has been investigated in this paper.Its effect on the cold cracking susceptibility is more important than that of the initial hydrogen content H₀ in the deposited metal.The residual diffusive hydroden H_R100 is equal to the effective diffusive hydrogen Hc without postheat,but when postheat is involved,Hc is equal to the residual diffusive hydrogen H_RP at the end of postheating.The calculating fromulas of the effective diffusive hydrogen Hc and the hydrogen diffusive factor D_F have been also put forward in this paper,and their computer programs have been compiled.Through the test of measurement,it was proved that the calculating formulas are belivable.The research has great significant for ascertaining the mechanism of welding cold cracking as well as for engineering application.