A fundamental-measure theory for a soft-sphere fluid has been extended to the study of the adsorption for a polydisperse soft-sphere fluid within a slit pore. The preferred species in a slit pore depends on the pore size and softness of a polydisperse soft-sphere fluid, but does not depend on the bulk packing fraction of a polydisperse soft-sphere fluid. For a fixed slit pore, the pore average mole fraction is nearly independent from the density development of a confined polydisperse soft-sphere fluid. The pore average mole fraction for small particles decreases linearly with increasing the pore size. The local relative concentration oscillates with a spatial period close to the diameter of a large particle the same way as the equilibrium particle density distribution.