Taking ZL104 aluminum alloy as the research object, the microstructure and mechanical properties of aluminum box girder with ultra-long laser cutting machine were studied by using metal mould low-pressure casting and sand casting. The results show that the yield strength and tensile strength of the beam and the base of the aluminum box girder cast by metal mold low-pressure casting are higher than those cast by sand mold, and the properties of the base are better than those of the beam under the same casting process. The grain size of the box girder samples cast by metal mold low-pressure casting is smaller than that cast by sand mold, and the microstructure is more uniform. The distribution of Si in sand casting sample is not uniform and there is a lot of segregation, while the distribution of Si in metal mold low-pressure casting is relatively uniform. Compared with sand casting, the secondary dendrite spacing in metal low-pressure casting box girder is about 60% lower and the mechanical properties are more than 20% higher.