Abstract: Compared with the traditional 2000 and 7000 series aluminum alloys, 1420 Al-Li alloy decreases density by 12% and increases Young's modulus(E) by 8%, and has excellent corrosion resistance and high temperature properties. Laser welding, as a fast and efficient welding method, has a serious porosity formation tendency in welding 1420 Al-Li alloy. The possible mechanism of porosity formation in laser welding 1420 Al-Li alloy were systematically analyzed. It indicated that the surface impurity is main hydrogen source and must be cleaned up thoroughly. Moreover, alloy elements such as Mg and Li, not only increase the hydrogen solution capacity in weld pool, but also increase the instability of keyhole tip. In order to minimize this instability and improve fluidflow in weld pool appropriate penetration mode should be employed.