Al−Ni−Er is an essential system in heat-resistant Al alloys. However, the aluminum-rich corner of this system, which has the most practical application significance, has not been fully studied. In this work, the phase equilibria of the Al−Ni−Er system are investigated via experiments and thermodynamic modeling. The isothermal sections of the Al-rich corner of this ternary system at 600 and 700 °C were determined through equilibrated alloys combined with scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffractometry (XRD). In addition, the vertical sections of the Al−Ni−Er system at Al0.7Ni0.3−Al0.7Er0.3 and Al0.8Ni0.2−Al0.8Er0.2 were measured via differential scanning calorimetry (DSC) analysis. A new ternary compound, τ14-Al12Ni2Er3, was discovered. On the basis of the experimental results of this and previous studies, the ternary Al−Ni−Er system was optimized with the calculation of phase diagrams (CALPHAD) method. The calculated isothermal and vertical section phase diagrams of the ternary system are in good agreement with experimental and literature data.