Abstract: The HAZ microstructure and mechanical properties of the 800 MPa grade ultra fine grained steel under different welding heat input were studied. The results showed that HAZ grain of the 800 MPa grade ultra fine grained steel had grown up distinctly under welding thermal cycle, and there was a more obvious growth trend along with the increase of welding heat input. HAZ contained ultra-low carbon bainite and thin ferrite, and the structure of overheated zone changed obvious under the effect of welding heat, which was the birthplace of the fracture cracks. By comparing to that of the base metal, the hardness value of the whole HAZ was decreased, and there were two distinct softened regions under different welding heat input. The yield strength of the welded joints was reduced gradually with the increase of welding heat input.