A combination of multichannel quantum-defect theory with the K reaction matrix has been used to imvestigate the influence of high-l 3pnl, (l=4) configuurations on the Mg 3pnd (j=3) autoionization spectra. The introduction of the 3Pug (J=3) configurations allows us to construct a complete model of the 3pug (J=3) autoionization process. Physical mechanism of some experimental phenomena have been related to the derailed structures of the autoionization spectra. Our theoretical results agree well with the published experimental spectra.