A four-level atomic system interacting with two laser fields and one probe field has been transformed into a four-level Dressed-Atom System interacting with a weak probe absorption field by means of the Dressed-Atom states. By using perturbation theory to solve the density matrix equations in Dressed-Atom representation, the analytic expressions for absorption spectrum are obtained. The properties of absorption spectrum are discussed in detail. Finally, the induced electric dipole moment between the atomic levels and its relation with the a.bsorption spec tram are deduoed.