In order to systematically study autoionization spectra of the Eu atom and discuss their spectral characteristics, the three-step isolated core excitation (ICE) is used in the present experiment. The atom is excited from its ground state via the 4f76s6p6P5/2 or 4f76s6p8P5/2 intermediate state to the 4f76sns (n=7 or 8) or 4f76s7d Rydberg state, from which it is further populated to the 4f76p1/2ns (n=7 or 8) or 4f76p1/27d autoionizing state. Not only the profound interaction between the 4f76p1/2ns (n=7 or 8) and 4f76p1/2nd autoionization Rydberg series revealed, but also the fingerprints of other series mixed in the 4f76p1/2ns (n=7 or 8) autoionization spectrum are identified. Moreover, the information of level energy, line width and line shape of these autoionizing states is reported, while the validity of ICE method is discussed.