Superalloy components are constantly evolving toward more complex structures and thin-walled designs, and misrun and microporosity potentially result from the casting process. In response to the above issues, ProCAST simulations and experiments were used to investigate the castability of the Mg microalloyed K492M superalloy, and the influence mechanism of Mg on fluidity and microporosity defects was also revealed. The results show that the addition of Mg can efficiently improve the fluidity of the K492M superalloy and reduce microporosity. The longest fluidity is obtained when the content of Mg is 0.01 wt.%, which is approximately 32.9% greater than that of the master alloy, and the microporosity decreases from 0.033% to 0.005%. The mechanism is that the solidification temperature range and viscosity are reduced in the Mg microalloyed K492M superalloy, which is conducive to improving the melt fluidity. Moreover, the size of the carbides and remaining liquid phase quantity increase, and the viscosity decreases, which is beneficial for reducing the microporosity.