This research is conducted to investigate the internal concave defects which are most likely to occur at 3~6 o′ clock position (for all- position root welding) during the laser metal active gas (MAG) arc welding process of X70 steel. The results indicate that both heat input and torch angle have a significant impact on the internal concave defects. Owing to gravity, the liquid metal will drop into the weld pool, and thus lead to internal concave defects in the back weld. By decreasing heat input from 3 to 6 o′ clock and with the upward inclined torch angle to 80°, the weld pool can be held by the shielding gas force and the arc force. In the way mentioned above, the internal concave defects are effectively controlled and better weld appearance can be achieved. In addition, it is found that the internal concave has little influence on the tensile properties of static load. While the internal concave enhances the strength of extension to more than 596 MPa, it significantly make the fatigue properties decrease.