A heat mode of laser penetrating of thick steel plates has been investigated and the relations between laser power and beam size as well as welding speed have been obtained through energy balance. Meanwhile, the laser beam quality has been discussed. The 4 mm, 6 mm and 8 mm A 3 steel plates have been welded by 6￣8 kW high power from an unstable resonator high quality laser in the experiment. The welding speeds were 3.2 m/min, 2.2 m/min, and 0.9 m/min respectively. The welding depth/width ratio of these samples were more than 2:1 while that of the 6 mm steel plate was 4:1. The technology of laser welding was emphasized. The testing of tensile strength and the experiment of rolling of steel plates through a rolling mill were carried out also.