A solute trapping model is developed based on a so-called solute drag treatment. By adopting a basic approach of phase-field models, and defining the free energy density in the interfacial region, a suitable interface shape function is introduced to derive the current model, in which the equilibrium and non-equilibrium interface behaviours can be described using a dimensionless parameter L (i.e. an important parameter in the present interface shape function). When applying the current model to Si-9%As (molar fraction) alloy with L=0.5, a good prediction of the steeper profile for high interface velocity, which is analogous to that using a phase-field model of DANILOV and NESLTER, has been obtained.