Abstract: Double-sided synchronization downhand-overhead welding (DSSDOW) of 5083 aluminum alloy with thickness of 8mm was conducted. The shaping mechanism of molten pool was analyzed, and the mechanical properties of joints were tested. The results show that "common pool" formed when the workpiece was completely penetrated. The depression of molten pool began when the collapsing force was larger than the maintaining force, and the extent of depression increased with the increase of heat input. The flexure fluid level produced additional pressure which was proportional to the extent of depression. The force in weld pool reached equilibrium finally. It meants the joint with concave-up convex-down shape formed, which was like an inverted arch bridge. The melting efficiency of DSSDOW was up to 17.29%, more than 2.20% of single side arc welding. The average tensile strength and elongation of the resultant joints were 275 MPa and 14.73%, which were 90.7% and 53.18% of those of base metal, respectively.