By means of analysing emission characteristics of plasmas produced by laser irradiation on Al planar target with different incidence angles, we find that: with the same laser irradiance intensity, the intensity of LPX (laser produced X-ray), which is produced by laser irradiation on Al planar target normally, is much stronger than that produced by laser irradiation on Al planar target obliquely;and the increase of the intensities of the H-like transition lines are much more obvious. In addition, the experimental results also show that the size of LPX source along the normal of the target surface, which is produced by laser irradiation on Al planar target obliquely, is much larger, and the widths of the spectral lines wider.