Abstract: Mechanical properties, fracture morphology and metallographic structure of 50 mm thick 5383 aluminum-magnesium alloy welded joints with different preheating temperatures were studied. The results showed that although it could be achieved good weld forming and excellent internal quality with lower preheating temperature, the mechanical properties were relatively lower. Analysis results showed that low preheating temperature led to low temperature of welding pool, so lots of FeMn rich phases in aluminum alloy could not melt completely. The remaining FeMn rich phase segregated, which caused the deterioration of local plasticity and strength of welded joints.