The superplasticity, gas bulging forming properties and microstructure of LZ91 Mg-Li alloy sheet were studied by hot tensile test, gas bulging forming experiment, metallographic analysis and scanning electron microscope observation. The results show that the rolled LZ91 alloy sheet exhibits excellent superplasticity: the elongation is up to 343.7% and the strain rate sensitivity index is 0.697 when the hot tensile temperature is 573 K and the strain rate is 0.001 s-1. Under the conditions of bulging temperature of 573 K and bulging pressure of 0.06 MPa, the sheet forming height is 51.14 mm and the height-diameter ratio is 1.279, which indicates that the Mg-Li alloy sheet has good superplastic forming potential. Dynamic recrystallization occurs in both hot tensile and superplastic gas bulging forming, it can effectively improve the plastic forming ability of the alloy. The typical superplastic cavities morphology at both tensile fracture and bulging fracture shows that the main deformation mechanism is grain boundary slip, and the main cause of superplastic failure is the growth and connection of the cavity.