Abstract: The laser pre-melting activating welding of low carbon steel with 8 mm thickness was study. Firstly, the surface of the substrate was pre-melted by the low power laser in oxygen atmosphere to form an oxide layer on the surface. Then TIG welding or laser arc hybrid welding was used to cover the weld.The effect of welding parameters on the weld depth and width was studied. The result shows that the arc was not shrunken during the welding after laser pre-melting. However the penetration increased about 0.5 times. The weld surface had a good appearance. The penetration increased with the increase of arc current, which had more increase after laser surface pre-melting. The increase of welding speed resulted in a decrease of penetration.The penetration increased with the increase of the pre-melting laser power. Simultaneously, the increase of laser power of hybrid welding also can increase the penetration. The oxygen content of the weld increased when the surface of weld was pre-melted by laser, which leads to the temperature coefficient of surface tension gradient changed from a negative value to a positive value.That is the reason why the penetration can be increased after laser pre-melting.