The continuum model proposed by the present authors for macroscopic solidification transport phenomena and macrosegregation formation was extended to account for the effect of any incomplete solid back diffusion through a microscopic dendrite solidification model. A generalized coefficient for the solid back diffusion was proposed by the present microscopic/macroscopic model, which includes the growing dendrite dimension, the parameters for the dendrite geometry, the solidification rate and partition coefficient etc besides the solid diffusivity.Numerical simulation of macrosegregations in a horizontal directionally soliditied A1-4.4Cu ingot with assumed different solid diffusivities of the copper solute was performed using the extended continuum model. The computation results show that the variation of the solid back diffusion can evidently influence the macrocomposition distribution in the soliditied ingot, and the greater the solid solute diffusivity, the lighter the macrosegregation seventy.