  Based on the frequency-domain theory, we investigate the above-threshold
ionization (ATI) process of an atom in a two-color laser field with infrared
(IR) and extreme ultraviolet (XUV) frequencies, where the photon energy of the
XUV laser is close to or larger than the atomic ionization threshold. By using
the channel analysis, we find that the two laser fields play different roles in
an ionization process, where the XUV laser determines the ionization
probability by the photon number that the atom absorbs from it, while the IR
laser accelerates the ionized electron and hence widens the electron kinetic
energy spectrum. As a result, the ATI spectrum presents a terrace-like
structure. By using the saddle-point approximation, we obtain a classical
formula which can predict the cutoff of each plateau in the terrace-like ATI
spectrum. Furthermore, we find that the difference of the heights between two
neighboring plateaus in the terrace-like structure of the ATI spectrum
increases as the frequency of the XUV laser increases.
