We report on the measurement of the L-shell absorption spectrum of Mg laser plasma. The resonance absorption lines of Li-like and Be-like ions are clearly observed. The electron density is sufficiently high so that our sample plasma is in nearly complete local thermodynamic equilibrium. At the determined electron temperature 56 + 4 eV and mass density 0.01 ± 0.002 g/cm3, the predicted transmission spectrum is calculated. The simulation result is in good agreement with the experiment.