Abstract: The Zn-Mg-Zr-B intermediate alloy prepared by traditional casting process was added to Mg-Y-Zn system long period magnesium alloy. The effect of Zn-Mg-Zr-B intermediate alloy on the microstructure and mechanical properties of as-cast Mg₉₄Zn_2.5Y_2.5Mn₁ alloy was investigated. The action mechanism was also discussed. The results show that Zn-Mg-Zr-B intermediate alloy can refine the grain size of as-cast alloy and promote the formation of 18R LPSO phase effectively. When the content of Zn-Mg-Zr-B intermediate alloy is 2wt%, the tensile strength and elongation of as-cast alloy can reach 226 MPa and 11.5%, respectively, and the properties are the best.