The LPSO structure reinforced magnesium alloy with low Y and Ni contents can achieve the best balance between performance and cost. To further improve the comprehensive mechanical properties, doping with Al element and ultrasonic vibration treatment are feasible approaches. The microstructure of low Y and Ni containing Mg₉₈Y_1.0Ni_0.5Al_0.5 alloy with Al addition and the effect of ultrasonic vibration on the microstructure and mechanical properties were studied by SEM-EDS, TEM, XRD and nano-indentation. Doping Al into the alloy decreases the amount of LPSO structure, and Al₂NiY phases with radial morphology precipitate near the block LPSO structure. The Al₂NiY phase is non-coherent with LPSO structure and Mg matrix at the phase interfaces. The Al₂NiY phase can be effectively modified into short flakes and distributed uniformly in the matrix with ultrasonic vibration treatment. The mechanical properties of Mg₉₈Y_1.0Ni_0.5Al_0.5 alloy are improved by reducing the generation of microcracks and preventing their propagation. Compared with Mg₉₈Ni_0.5Y_1.0Al_0.5 alloy without ultrasonic vibration treatment, the ultimate tensile strength and elongation of the alloy with ultrasonic vibration treatment are improved to 187 MPa and 7.9%, with increments of 21.4% and 105.7%, respectively.