Strip casting process is a kind of promising near net shape technology for producing magnesium alloys strip. The microstructure and mechanical properties of CO2 laser welded as-cast AZ31 magnesium alloy sheet were investigated. The microstructure of base material (BS) was typical columnar dendritic structure. A significant grain refinement occurred in the fusion zone (FZ) due to the rapid cooling, while an obvious grain coarsening occurred in the heat affected zone (HAZ). The tensile strength and elongation at fracture of weld joints were reduced due to the weld depression caused by high heat input. The hardness in the FZ was the highest due to fine grains, while there was a decrease in hardness of the HAZ, but little variation compared with the hardness of the BS.