The present study is to improve the sensitivity of detection and reduce the limit of detection in detecting heavy metal of soil by laser induced breakdown spectroscopy(LIBS).The Cr element of national standard soil was regarded as the research object. In the experiment, a conical cavity with small diameter end of 20 mm and large diameter end of 45 mm respectively was installed below the focusing lens near the experiment sample to mainly confine the signal transmitted by plasma and to some extent to confine the plasma itself in the LIBS setup. In detecting CrⅠ425.44 nm, the beast delay time gained from experiment is 1.3 μs, and the relative standard deviation is below 10%. Compared with the setup of non-spatial confinement, the spectral intensity of Cr in the soil sample was enhanced more than 7%. Calibration curve was established in the Cr concentration range from 60 to 400 μg·g-1. Under the condition of spatial confinement, the liner regression coefficient and the limit of detection were 0.997 71 and 18.85 μg·g-1 respectively, however, the regression coefficient and the limit of detection were 0.991 22 and 36.99 μg·g-1 without spatial confinement. So, this shows that conical spatial confinement can improve the sensitivity of detection and enhance the spectral intensity. And it is a good auxiliary function in detecting Cr in the soil by laser induced breakdown spectroscopy.