Compared with the traditional sieve-pipette method, the laser diffraction method is a new technology for determination of soil particle size distribution, which has obvious advantages in measurement efficiency and operation error reducing. However, the laser diffraction method always underestimates the clay content. In order to set up the conversion models, 70 soil samples selected from karst areas of Northwest Guangxi were measured by Mastersize2000 laser diffraction and the sieve-pipette method. The results showed that based on the classification criteria of USDA (United States Department of Agriculture), the clay volume content (< 0.002 mm) using the laser diffraction method was lower than that using the sieve-pipette method, while the silt volume content (0.002 mm - 0.05 mm) was higher. The relative errors of the clay and silt contents increased with the increase of clay content measured by sieve-pipette. The sand content was sometimes higher and sometimes lower with the lowest relative error. Significant relationships were found between the clay and sand contents determined by the two methods, but not between the silt content. However, the < 0.02 mm content measured by the two methods was significantly correlated. The laser diffraction measured results of soil particle size distribution could be converted by the linear regression equation of the following four particles grades including < 0.002 mm, < 0.02 mm, 0.02 mm- 0.05 mm and > 0.05 mm.