  We propose a realization of pure spin currents and perfect valley filter
based on a quantum anomalous Hall insulator, around which edge states with
up-spin and down-spin circulate. By applying staggered sublattice potential on
the strips along the edges of sample, the edge states with down spin can be
pushed into the inner boundaries of the strips while the other edge states with
up spin remain on the outer boundaries, resulting in spatially separated chiral
states with perfect spin polarization. Moreover, a valley filter, which is
immune to short-range and smooth long-range scatterers, can be engineered by
additionally applying boundary potentials on the outmost lattices of the
sample. We also find that the boundary potential can be used to control the
size effect induced oscillation of the inner chiral states. The connection of
the boundary potential to size effect is revealed.
