Understanding and revealing the evolution mechanism of the columnar to equiaxed transition (CET) that occurred in the medium-entropy alloys (MEAs) fabricated by laser directed energy deposition (LDED) is crucial for achieving the tunable microstructure and mechanical properties. In the present work, a CET map was established to explain the effect of the laser energy density (LED) on the CET of the LDED fabricated single tracks of the CoCrNi MEA. The results show that the CET observed in single tracks is related to the morphological transformation of cellular substructure, which is governed by constitutional undercooling. With the increase of LED, the maximum value of the constitutional undercooling exceeds the critical nucleation undercooling earlier, which facilitates the heterogeneous nucleation and inhibits the epitaxial growth of rod-like cells, and finally causes a promoted CET. The correlation among the average cell size, geometrically necessary dislocation density, and average hardness of single tracks was also discussed. The present work not only uncovers the mechanism of the CET and grain growth in the LDED-fabricated CoCrNi MEA but also provides theoretical guidance for fabricating other MEAs with ideal microstructure and performance.