In the event of a severe accident, when the β-phase in the cladding melts, the molten zirconium alloy begins to dissolve with the UO2 and ZrO2 oxidation layers, forming a (UO2, ZrO2, Zr) molten mixture. This reaction can lower the material melting points, thereby affecting the core melting process. Severe accident analysis software typically uses the positions of the solidus or liquidus lines in the U-Zr-O ternary phase diagram to determine whether the dissolution reaction continues, and thus to ascertain the dissolution limit of the (UO2, ZrO2, Zr) eutectic mixture. Therefore, the determination of the solidus or liquidus lines in the U-Zr-O ternary phase diagram is crucial for studying the core melting process following the reduction of material melting points. In this work, the CALPHAD method was employed, integrating reported iso-thermal section data and thermodynamic properties to establish binary phase diagram databases for Zr-U, O-U and Zr-O. Based on these databases, the U-Zr-O ternary phase diagram was extrapolated. Subsequently, the isothermal section phase diagram of the subsystem UO2-ZrO2-Zr at 1273–2673 K was calculated from the U-Zr-O ternary phase diagram. The phase diagram reveals that at temperatures around 2273 K, the liquid phase region first appears in the Zr-rich area, and as the temperature increases, the liquid phase region extends towards the UO2- and ZrO2-rich areas.