To obtain the higher quality of the laser-induced breakdown spectrosco py (LIBS) and improve the detection capability of the LIBS technique on soil samples,the effect of fl at mirror on the radiation of the laser-induced plasma is investigated by changing the distance between the flat mirror and the axis of the plasma center.And,the plasma parameters,the electron temperature and electron density of plasmas,are measured by the Boltzmann grap h method and Stark broadening to explain the mechanism of the radiation enhancement obtained with the flat mirror.The experimental results indicate that the plasma radiation intensity is significantly enhanced with the mirror,and the plasma radiation intensity is gradually weakene d as the distance increasing.The calculated results show that the spectral line intensit ies of elements Cr,Fe and Ca with the distance between the mirror and the axis of the plasma center of 3mm are enhanced by about 49.3%,50.1%, and 75.9%,respectively than those without mirror,the spectral signal-to-noise ratios are increased by about 56.8%,47.5%,and 65.5%,respectively while the electronic temperature and electronic density of pl asmas are enhanced by 1250K and 0.62×10¹⁶ cm－3,respectively.It is an effective met hod to obtain the high-quality laser spectrum.