To reduce the influence of self-absorption of laser plasma emission spectra and improve the detection capability of laser-induced breakdown spectroscopy to material component,a plane mirror device was used to constraint the laser plasma.The spectral line profile of Mn and Ni in steel samples under different experimental condition were studied and analyzed quantitatively.The experimental results demonstrate that the self-absorption of spectral line decreases obviously when a plane mirror device is used.Through quantitatively analyzing Mn and Ni,once inserting a plane mirror device,the value of relative standard deviation of Mn decreases from 3.70% to 1.86%,and that of Ni decreases from 6.23% to 2.16%,which indicates that the measurements precision is improved obviously.It is thus clear that the plane mirror device could reduce the adverse effect of self-absorption on analyzing results obviously.