Space resolved spectra with both high spectral resolution (-0.2A) and spatial resolution (-2μm) of the second harmonic emission scattered at 90°to the laser axis were observed when both narrow band and broad band laser beams were adopted in an Al planar target experiment. After analyzing many phenomena, it has been proved that the second harmonic emission observed at 90° to the laser axis is initiated by the interaction of laser with plasma filaments other than that of planar wave with plasmas. And theory is in agreement with experiment on the whole.