Regarding to the inadequacy of traditional soil nutrition detecting techniques, a new soil nutrition on-line detecting equipment based on laser induced breakdown spectroscopy theory is developed. The experiments to analyze feasibility and accuracy of detecting soil potassium nutrient content is carried out with this equipment. Experimental results show that characteristic lines of potassium are 766.49 nm and 769.90 nm respectively, and the relationship of soil potassium nutrient content and characteristic peak value is positive correlation. However, once potassium content in samples is above 0.3%, the relationship of potassium nutrient content and characteristic spectral intensity becomes nonlinear because of the existence of emission spectrum self-absorption phenomenon. To eliminate the detecting matrix effects, lithium element in soil samples is selected to be a internal standard element for correcting potassium element characteristic lines. Using spectral intensity of sensitive characteristic wavelength 766.49 nm and 769.90 nm to estimate soil potassium nutrient content by binary linear model, the determination coefficient (R2) of model reaches 0.933 7, and the relative root mean square error(RRMSE) is only 0.276 1. With current testing conditions, the lowest detection limit of soil potassium content is 212 μg/g. This result indicates that the detecting system can satisfy the requirements of soil potassium nutrient on-line detection preliminarily, but needs to be further improved.