The cavity field spectrum of a cascade three-level atom interacting with a single-mode field in an ideal cavity is investigated. When the atom is in the upper level and the field is vacuum initially,the number of Rabi peaks changes according to 2→6→4→2→4 with the increase of R=g2/g1,and all Rabi peaks disappear for R1 Three-peak,five-peak or seven-peak structure appear for weak initial fields,and single resonance peak appears for a st rong field. When the atom is in the middle level initially,the cavity field spectrum is of a two-peak structure with R=1 as the spectrum of Jaynes-Cummings model.