To improve the quality and the detection capability on soil samples of laser induced breakdown spectroscopy, the effect of different heights of carbon sheets from the target surface on the soil plasma radiation intensity is investigated. Moreover, the electron temperature and electron density of plasmas are measured through the Boltzmann graph method and Stark broadening. Experimental results indicate that the plasma radiation intensity is significantly enhanced when the plasma is confined by carbon sheets, compared with no carbon sheets confinement. As the height of carbon sheets from the sample surface increases, the plasma radiation intensity gradually increases, peaks at 11 mm and then decreases. The calculated results show that the spectral line intensity of elements Fe, Mn, K and Ti with carbon sheets confinement at 11 mm from the target surface are enhanced by about 179.88%, 117.02%, 123.21% and 91.24%, respectively, than those without carbon sheets confinement; spectral signal-to-noise ratios are increased by about 107.30%, 92.26%, 68.48% and 67.66%, respectively; the electronic temperature and electronic density of plasmas are increased by 2800 K and 2.16×1016 cm-3, respectively. It provides a simple and easy method for the detection of trace elements in soil.