Abstract: The modification test of LZ91 magnesium lithium alloy was carried out under the process parameters of 1600r/min, 1800 r/min and 1900 r/min, respectively. Optical microscope, universal testing machine and microhardness tester were used to analyze the effect of rotation speed on the microstructure and mechanical properties of the samples. The results show that when the advancing speed is 60 mm/min and the rotation speed increases from 1600 r/min to 1900 r/min, the grain size ofα-Mg phase and β-Li phase in the nugget area and the thermo-mechanically affected zone on the advancing side increases,however, the grain size of α-Mg phase and β-Li phase in the thermo-mechanically affected zone on the retreating side first decreases and then increases. After friction stir processing modification, the tensile strength, elongation and microhardness of nugget zone of LZ91 magnesium lithium alloy significantly improves, which show a trend of first increasing and then decreasing. When the rotation speed increases to 1800 r/min, the tensile strength reaches the maximum value of 252.0 MPa,and the tensile strength increases by 36% compared with that before friction stir processing modification.