Abstract: The grain growth and phase transformation in HAZ of TiNbV microalloyed steel under simulated welding thermal cycling were studied by in situ observation method using laser confocal microscope. The results show that the transformation from ferrite and pearlite to austenite is occurred while the heating temperature rises to 860 ~ 980 ℃ during thermal cycling. The austenite grain begins to grow obviously while the temperature reaches 1 100 ℃. At the temperature range of 1 300 ~ 1 400 ℃, the grain grows rapidly with the form of combining. During cooling process, when the temperature declines to 1 400 ~ 1 350 ℃, the grain grows slowly with the form of grain boundary migrating. When the temperature decreases to 660 ~ 580 ℃, the austenite transforms rapidly to bainite. The content of HAZ is mainly composed of bainite and ferrite. The size of austenite grain determines the maximum size of bainite. The formation, transformation and end temperature range of austenite and bainite decreases with the prolongation of the high temperature residence time. The content of proeutectoid ferrite decreases first and then increases, the content of bainite decreases, the polygonal ferrite disappears, and the cooling structure tends to be uniform and coarse. During the welding process, the choosing of appropriate high temperature residence time can increase the content of IAF in HAZ and improve the mechanical properties of the joints.