The phase transformation path is vital to enclosing the macroscopic transport equations for predicting alloy macrosegregation. However, the analytical approximations for micro-segregation, such as the lever rule (LR), are invalid because an actual alloy is a multi-component system with several coexisting solids. The LR only expresses the phase transformation between a single solid phase and a liquid phase and adopts a constant solute partition coefficient, which is insufficient for micro-segregation. In this study, a model combining the thermodynamic phase transformation path calculation with the macroscopic transport was adopted to predict the macrosegregation formation in an Fe-0.1%C (mass fraction) peritectic alloy, which considers the coexistence of multi-solids and the variance of the local partition coefficient at the solid/liquid and solid/solid phase interface with a solidification process. The phase transformation path from the liquid state cooling to room temperature within a certain range of the solute concentrations was obtained using the LR approximation combined with the thermodynamic equilibrium calculation (LR-TEC). By tabulating the phase transformation path and interpolating the local concentration and enthalpy, the corresponding temperature, phase fraction, phase concentration, and phase enthalpy required in the continuum macroscopic transport model were achieved. The latent heat released and the specific heat corresponding to the amounts of the two solid phases at the peritectic or eutectic phase transformation zone were updated along with their dependence on the local concentration and temperature. This method was validated through the benchmark macrosegregation test of the binary Sn-5%Pb alloy. Regarding the Fe-0.1%C alloy, the varied local partition coefficients and the other thermodynamic parameters with multi-solids precipitating during solidification resulted in a more severe macrosegregation profile in the ingot. At the end of the solidification calculation, the predicted minimum relative solute concentration for the Fe-0.1%C alloy was -2.22% at y = 45 mm from the bottom and x = 16 mm from the left wall of the ingot by LR-TEC. In contrast, it was -1.78% using the LR Analytical model near y = 55 mm at the left-side wall. The predicted maximum macrosegregation ratio at the right wall of the ingot by LR-TEC was 1.13% larger than that achieved using the LR Analytical model. Several solids, such as α and γ, α and cementite (CEM), or α, γ, and CEM at the left part (x < 0.0342 m), and δ and γ at the right (x > 0.0858 m), still coexisted in the region at the end of solidification calculation.