In this paper, a Zn-Mg-Ti master alloy was prepared by mould casting, and the effect of Zn-Mg-Ti master alloy on the microstructure and mechanical properties of pure magnesium were studied. The results showed that the master alloy was mainly composed of matrix and flower-like Zn-Mg-Ti ternary phase, and that the Zn-Mg-Ti master alloy had a significant influence on the microstructure and mechanical properties of pure magnesium. The grain size of magnesium alloy decreased first and then increased with the increase of master alloy addition; when the addition amount of the master alloy was 8%, the finest grain size could be obtained. The grain refinement of the magnesium alloy is mainly attributed to the Ti atoms that agglomerate at the front of the solid-liquid interface and thus restrict the crystal to grow. Compared with the microstructure of the Mg-6.4wt.% Zn alloy and M-8 alloy, it was found that Ti element could not only significantly refine grain size of Mg-Zn alloy, but also promote the dissolution of the second phase into the matrix. The mechanical properties of as-extruded alloys showed that the strength and elongation of magnesium alloy increased first and then decreased with the increase of addition amount of the master alloy; when the content of the master alloy was 8%, the ultimate tensile strength and elongation of the magnesium alloy were 308 MPa and 21.5%, respectively.