All the perturbed Rydberg series, i. e., 3sns 1S0,3snp 1P1O, 3P0,1,2O,3snd 1D2 and 3snf 3F2,3,4O, have been studied by the multichannel quantum defect theory (MQDT). The analysis show that, the first two series are weakly disturbed, the rest, however, are strongly perturbed by the core-excited states: 3p4s 1P1O,3p2 1D2 and 3p3d 3F2,3,4O, respectively and 3snf 1F3O by 3p3d 1F3O which may be located 5613 cm-1 above the first ionizing potential according to our estimation. This prediction agrees with those made by other authors.