In the present work, spatial character of emission spectrum was analyzed, the effect of laser energy and samples attribute on the best detection position for the highest signal-to-noise ratio wass tudied, and some experimental investigations with LIBS technique to detect trace Cu in polluted soil were carried out in our laboratory. A Q-switched Nd∶YAG laser operating at 1064 nm with pulse width of 10 ns and repetition frequency of 1 Hz was utilized. The laser pulse was focused by lens with focal length of 10 cm togenerate microplasmas on the surface of printed circuit board and soil samples. The sample was adjustable by vernier construction to detect the emission spectrum of the microplasmas from different position. Experiments showed that the intensity of thermal radiation and atomic radiations evolved differently while the detection position changed. It was verified that thermal radiation reduced rapidly with the distance from the center of spark increasing, while the intensity of atomic radiations increased firstly and decreased after intensity maximum was reached. The method of separating thermal radiation and atomic radiations in space brought on high signal-to-noise ratio. It was found that the best detection position was 0.75 mm off the center of the spark for soil sample while the laser energy was 40 mJ, and the distance increased with the growth of laser energy.With Cu 324.75 nm and Cu 327.39 nm as the analysis lines, the best detection position was selected to detect trace Cupollution in soil. Internal standard method was used to determine the relation between Cu concentration and its intensity. It was concluded that the detection limit of Cu in soil was 67 mg·kg^-1, which is below the trace element thresholds for Class 2 soil defined in the Environmental Quality Standard for Soil in China. It was proven an effective way to achieve higher signal-to-noise ratio by adjusting the location of spectral measurements. This method was viable for trace Cu detection in polluted soil.