Abstract: In this paper the rule of preheating and postheating on both cold crack sensitivity and toughness of the HAZ of 30CrMnSiNi2A low alloy heat treated high tensile steel was investigated by means of the implant test method.The results show when preheating at 250℃ and postheating at 260℃ 1h,then σ_cr>σ_a.The fracture toughness of specimens under different weld heat cycles was evaluted by means of the weld thermal simulation method.It was discoverd that by the implant test and the weld thermal simulation method may lead to optimize the welding parameters.The micro structure and distribution form of residual austenite of HAZ were observed by means of TEM.The amount of the residual austenite was evaluated.It was discovred that the micro-structure of HAZ was mainly constituted from dislocated lath martensite,and somelower bainite and 9.8% residual austenite by taking.the preheating at 250℃ and post-heating at 260℃ 1h.The residual austenite distributes between the martenite lathes and in the form of thin ilm.It toughens structure and may prevent the diffuson of hydrogen as well.Therefore it may be beneficial from the view of preventing the occurrence of cold crack.The experimental results offer an exact fundamental basis for applying the optimum postheating technology and giving an explainasion about the toughening and anti-cracking effect in the HAZ by postheating for this type of steel.