Abstract: In order to reduce the self-absorption effect of spectral lines in the laser induced plasma emission, which could improve the detection level of laser spectral analysis technology for high-content elements in materials, a plane mirror device was used to constraint plasma in the experiment. The change of the spectral lines with or without the plane mirror device was compared. Without the plane mirror device, the half-maximum line widths of spectral line of the sample element Al, Mg and Mn were 0.16 nm, 0.24 nm and 0.058 nm, respectively. But they were 0.11 nm, 0.13 nm and 0.047 nm with four plane mirrors device constraining plasma. It shows that the self-absorption effect of spectral lines decrease significantly by the use of plane mirror device to restrain plasma on the space, while the spectral line becomes sharper and the intensity is improved significantly. By observing the plasma photos and measuring of plasma temperature and electron density, the reason to reducing the self-absorption effect in laser spectrum is discussed.