In order to improve the spectral characteristics of laser-induced breakdown spectroscopy (LIBS),a LIBS detection system under the constraint of planar mirrors was built.The planar mirrors were placed on both sides of the sample.Planar mirror spacing of 7 mm,9 mm,11 mm and 13 mm were selected to carry out the experiment.It was obtained that the plasma radiation intensity decreases with the increase of the plane mirror spacing.The effects of different plane mirror spacing on the plasma properties of Fe,Al and Pb elements in soil samples were studied.The experimental results show that: compared with no plane mirror constraint,when the plane mirror spacing is 7 mm,the signal-to-noise ratio of the three elements Fe I,Al I,and Pb I in the sample is increased by 29.9%,39.4%,and 31.0%,respectively.The plasma temperature was increased by 484.54 K,and the plasma electron density was increased by 2.41×1015 cm－3.The detection limit was reduced from 86.9 mg/kg to 51.2 mg/kg and the relative standard deviation (RSD) was reduced from 7.8% to 4.6%.It can be seen that the use of planar mirrors is a simple and effective way to improve the spectral characteristics of laser.