Experiments of frequency-tripled laser beam on the Al flat target are carried on the XINGGUANG Ⅱ laser facility. A flat-grating spectrometer is used to measure Al plasma X-ray in the range of 3 nm to 6 nm. Based on relativistic multi configuration interaction, and taking account of CI interaction and correction of Berit, L shell lines have been computed by Cowan codes. A total of 22 spectral features are identified, and calculations agree with experimental emission spectra. Results show that identified lines belong to transitions from 2s, 2p of L shell to 3s, 3p shell, even higher shell, and maximum difference between our measurements and theoretical results is only 0.06 nm. The results of oscillator strength and other theoretical results are consistent.