Abstract: The new generation steel will be the main structural material in 21st century. The MAC pulsed welding,one of the conventional fusion welding,can suit to the new generation steel. In order to promote the research on its weldability and engineering application, the weld thennal cycle equation is described correctly according to the measured MAC pulsed weld thermal curves. And then the HAZ austenite grain growth equation and diagram of the new generation steel with yield strength 400 MPa are established. The grain growth in all parts of HAZ can be predicted through the diagram,which reflects its heat sensitivity,being one kind of its weldability. The conclusion of investigation is that the prediction data of the grain growth diagram mate well with the experimental data.