Abstract AZ91 magnesium alloy is extensively utilized in aerospace, automotive, and electronics industries because it is lightweight and has high specific strength and stiffness. However, its limited mechanical properties hinder its broad application. To enhance the comprehensive mechanical properties of AZ91 magnesium alloy, this study prepared AZ91 magnesium alloy by adding the rare-earth element lanthanum (La). Phase composition was examined through X-ray diffraction, and the microstructure was observed using an optical microscope and a scanning electron microscope. Mapping and energy spectrum analysis were performed to investigate the composition of the second phase. Differential scanning calorimetry was utilized to analyze the phase transition temperature, and the mechanical properties were tested. The solidification process was further assessed using Thermo-Calc simulation software. Results revealed that after the addition of La, a needle-shaped Al11La3 phase appeared in the microstructure. As the amount of added La increased, new granular Al11La3 and Al8LaMn4 phases emerged. When the amount of added La increased to 0.4 wt.%, the alloy exhibited maximum values for yield strength, tensile strength, and elongation values of 103.5 MPa, 169.75 MPa, and 4.4%, respectively. These improvements represent increments of 31.3%, 66.3%, and 214.3% in yield strength, tensile strength and elongation, respectively, compared with the alloy without La. When the La content reached 1.6 wt.%, hardness achieved its maximum value of 68.1 HV, showing an increase of approximately 26%. The evolution of microstructure and strengthening mechanism of mechanical properties were discussed.