The CALPHAD method was used to optimize and calculate the phase diagrams of Nd-Dy, Dy-B binary systems and Fe-B-Dy, Dy-Fe-Nd and B-Dy-Nd ternary systems. The thermodynamic databases of Nd-Fe-B-Dy quaternary alloys were established by the thermodynamic parameters of binary and ternary sub-systems reported in the literature and took into account the consistency of the thermodynamic models. The longitudinal cross-section phase diagrams of Nd-Fe-B-Dy quaternary alloy Nd16-xDyxFe77B7 (x≤5 at.%) were predicted by using these databases. At the same time, the variation of the fractions of each phase of the permanent magnet alloy with different Dy content under equilibrium solidification was analyzed. These results could be helpful for the microstructure design of the Dy-doped NdFeB permanent magnet alloy and the selection of the preparation process parameters.