The demand for large format round steel billet has been increasing in past decades. As a new casting method, semi-continuous casting (SCC) process, which attempts to combine the merits of ingot casting and continuous casting techniques, is becoming a competitive technique in modern steel plants to produce large round. However, understanding to the process regarding to the formation of as-solidified structure, macrosegregation, porosity, and so on is not very clear. The objective of this work is to use a three-phase columnarequiaxed solidification model to simulate solidification process of SCC, evaluating the aforementioned issues and supplying instructive suggestions to the industry. This numerical model is originally developed by Wu and Ludwig [Metall. Mater. Trans. A, 2006, p.1613]. In order to model the SCC process, the numerical model was extended. Verifications of the extended model were made in several cases at different scales from laboratory benchmark to practical SCC strand.