Abstract: Autogenous laser welding of aluminum-lithium alloy often results in bad weld appearance and serious porosity. This paper investigates the influence of different laser welding positions on the welding quality of 5A90 Al-Li alloy. The weld appearance and the porosity are compared with the flat welding, the horizontal welding and the vertical welding respectively. The formation and suppression mechanism of defects in different welding positions are analyzed and discussed considering the force state of the melt pool, the liquid metal flow and the keyhole behavior. The experimental results show that the weld seams suffer from excessive penetration in the flat welding, and serious metallic spatters, discontinuous weld bead in the horizontal welding, the concave defects in the back weld in the vertical-up welding. The optimal weld appearance and least porosity defects could be obtained in the vertical-down welding position. The porosity is closely related to the welding positions due to the change of dynamic characteristics of the keyhole and the flow behavior of the melt pool. The porosity changes in a similar trend with the increase of the heat input in different welding positions. The pores increase at first then decrease, and a maximum value is reached when the weld is incompletely penetrated. The number of pores and their average size could both be suppressed into minimums in the vertical-down welding, and the pores formed mainly in the centerline of the seam while they distribute chaotically around the weld seam in other welding positions.