Mg–1Al, Mg–0.9Y, Mg–2Gd, Mg–1.5Zn, and Mg–0.5Ca binary alloys were chosen to clarify the hot tearing mechanism since they exhibited the highest hot tearing susceptibility (HTS) among each binary alloy systems. Hot tearing tests of these five binary magnesium alloys were carried out by a constrained rod-casting apparatus. 3D X-ray tomography technology was used to characterize their tear volume, tear patterns, and propagation paths of such tears. The results showed that the HTS of these five alloys followed Mg–1Al < Mg–0.9Y < Mg–2Gd < Mg–1.5Zn < Mg–0.5Ca order. Among them, Mg–0.5Ca alloy exhibited abnormal HTS because of its susceptible temperature range and residual liquid fraction. In addition to the previous analysis of hot tearing behavior, discussion from the point of view of fluidity, thermal expansion coefficient, and mechanical property of semi-solid alloy with high solid fraction were incorporated to clarify the hot tearing mechanism.