Square-well fluid under the influence of an external field is investigated by means of density functional theory. The attractive part of the square-well potential is treated by the mean field approximation, while the repulsive part is approximated by the hard sphere potential of the same bulk density with the investigated square-well fluid. When the square-well fluid is confined due to an external field, the hard sphere model fluid, approximating the repulsive part, becomes nonuniform and is then treated by a recently proposed density functional approximation by the present author (New J. Phys. 2002, 4, 36). The associated mixing parameter is determined in the present case by the hard wall sum rule for the square-well fluid. The prediction accuracy for the density distribution by the present work is comparable with that of a recently proposed weighted density approximation plus a third-order perturbation expansion approximation in the low bulk density case and is higher than that in the higher bulk density cases. The greatest advantage of the present approach is that the required input function is the bulk Percus-Yevick hard sphere direct correlation function, which is conveniently available. Thus numerical solution of the Ornstein-Zernike equation for bulk square-well fluid is avoided. Evidence for the continuity and differentiability of the density functional C-(1)(r;[rho]) is given.