The transformation behavior and microstructure development in the heat affected zone(HAZ) of 800MPa grade ultra fine structured steel was investigated. It was found thatthe HAZ has intermediate temperature transformation characteristics in a wide rangeof cooling rates, with the bainite sheaves consisting of bainite ferrite plates withoutcarbide precipitation and retained austenite in the fast cooling regime. At relativelyhigh cooling rates, which corresponded to low heat inputs, the hardness of the simulatedHAZ was above that of the base metal. When the cooling rate was below 9℃/s, thewelding HAZ would have an obvious softening. The analysis of transformation ratesin continuous cooling processes was completed by numerical differential method. Theresult indicated that the microstructure transformation rate of the HAZ in 800MPagrade ultra fine structured steel changed sharply to slow speeds when the cooling timet_(8/5) is longer than 7s. The founding of the HAZ has intermediate temperature transformation characteristics in a wide range of cooling rates, with the bainite sheaves consisting of bainite ferrite plates Withoutcarbide precipitation and retained austenite in the fast cooling regime. At relatively high cooling rates, which are corresponded to low heat inputs, the hardness of the simulated HAZ was above that base metal. When the cooling rate was below 9 ° C./s, the welding HAZ would be The analysis of transformation rates are continuous cooling processes was completed by numerical differential method. Theresult indicated that the microstructure transformation rate of the HAZ in 800 MPagrade ultra fine structured steel changed sharply to slow speeds when the cooling timet_ (8/5) is longer than 7s.