The phase equilibria of the Cu-Nb-Si system were investigated via a combination of key equilibrated alloys, thermodynamic modeling and first-principles calculations. Sixteen ternary alloys were prepared to determine the isothermal sections at 600 and 700 °C, by means of X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). The three- and two-phase regions were determined. The existence of ternary compound τ₁ (Cu₄Nb₅Si₄) was confirmed. The solubilities of Cu in the NbSi₂ and Nb₅Si₃ phases were measured. Based on the experimental equilibria data from the literature and the present work, a thermodynamic description of the Cu-Nb-Si system was carried out by using the calculation of phase diagrams (CALPHAD) method supported by first-principles calculations. The substitutional model and sublattice model were employed to describe the solution phases and intermediate phases, respectively. A set of self-consistent thermodynamic parameters of the Cu-Nb-Si system were conclusively obtained. Most of the reliable experimental data were reproduced by the present thermodynamic modeling.