The Multichannel Quantum Defect Theory (MQDT) has been applied to the analysis of the 2D absorption spectrum of atomic aluminium. The model consists of the mixing of three dissociation channels with different ionization thresholds. By suitably taking the values of the eigen-parameters μα,Dα which depend on electron energy and on the transformation matrix elements Uiα, the 2D autoionization spectrum between the first and second ionization limits is calculated and the result is compared with Roig's measurements. Extending the se parameters to the region of discrete spectrum, we obtained the Rydberg series of 2D energy levels as well as their oscillator strengths which also coincide with the analyses of Lin and Weiss and related experiments.