Abstract: For understanding the microstructure acicular ferrite(AF), the thermal simulation test of weld metal in X70 pipe-line steel was carried out with GLEEBLE 1500.SEM and TEM were used to analysis the relations between the inclusions and the AF, and the C concentration distribution along AF boundary and the characteristics of sympathetic AF sufferred from secondary heat cy cling.The results show that there exits a layer of thin C-riched film along the AF boundary, and both the shear mechanism and the C diffusion were occurred during AF growth.The sympathetic AF nucleated on the site of high density dislocation of the primary AF and grown into specific size rapidly.Both the AF and sympathetic AF were nucleated and grown-up in the austenite grain, and were good to fine-grained strengthening effect in the multi-pass weld metal and HAZ.