<p>Columnar to equiaxial crystal transition (CET) is an important technological feature in many casting processes. This work investigated the CET during the solidification of Mg-Gd-Zn alloys by combining synchrotron radiation in-situ imaging and phase-field method. Results show that the grain size, dendrite tip radius, and secondary dendrite arm spacing (SDAS) all exponentially decrease with an increase in cooling rate (<i>V</i>_c). The variation in the radius of the dendritic tip is similar to the prediction of the Hunt model, while the variation in the SDAS is close to the Bouchard-Kirkaldy model. It is worth noting that the CET is promoted by a decrease in the temperature gradient (<i>G</i>) and an increase in the cooling rate (<i>V</i>_c). In both equiaxed and columnar crystal regions, the dendrite tip growth rate and solid phase volume fraction increase with increasing <i>G</i> and <i>V</i>_c. In addition, the CET process has been predicted by simulation. The results are consistent with the predictions of the GTK model, which is important for the in-depth study of the dendrite morphology in different crystallization regions. In the final stage, the effects of different critical subcooling degrees and nucleation densities on the CET were explored. The results show that increasing the critical nucleation supercooling degree can inhibit the generation of equiaxial crystals, while increasing the nucleation density helps to promote the CET.</p>