Abstract: The large thickness Ti6321 titanium alloy welding was studied. The feature of location and morphology of root defects was observed. The reason of root defects was analyzed and the mechanism of root defects was revealed. With the results of the test, some methods were proposed to reduce and eliminate the root defect. The results show that for large thickness titanium alloy welding, penetration welding was prone to producing the spiking defects and porosity defects. When the surface tension near the bottom of weld pool keyhole was smaller than that of the metal vapor pressure, it was hard for the surface tension of the root to maintain the stability of key hole wall, then liquid metal collapsed to the keyhole. Metal vapor cannot escape under rapid cooling conditions, so the liquid metal solidified and formed spiking defect, namely root incomplete fusion and root holes. When the distance from the equilibrium position of the surface tension and the metal vapor pressure to the bottom of the keyhole were greater than half of penetration, weld root was prone to chain porosity defects. Taking measures to match vapor pressure and surface tension or completely penetration to the back can effectively reduce and eliminate the spiking defect and chain porosity defects to obtain a good weld.