The influence of impurity content on the microstructure and mechanical properties of ZK60 magnesium alloys was investigated by optical microscopy, scanning electron microscopy and tensile test. ZK60 alloys were prepared by changing holding time of alloy melt during semi-continuous casting in order to control the content of impurity elements. The alloy with lower purity content is found to have less second precipitates and larger grain size in the as-cast state. However, in the as-extruded state, reducing impurities brings about a decrease in grain size and an increase in yield strength from 244 MPa to 268 MPa, while the elongations in the as-extruded alloys with different contents of impurities are almost the same. After T5 treatment, impurity content is found to have more obvious effect on the yield strength of ZK60 alloy. The yield strength of ZK60-45 alloys with low impurity content is increased up to 295 MPa after T5 treatment.