In order to study the particle size distribution of fine grained soil, the Malvern Mastersizer 2000 laser diffraction grain size analyzer and traditional densimeter method were used to test the clay in Hefei. According to the work principle and affecting factors of laser method, the parameters, such as the refractive index, concentration, ultrasonic displacement and time, pumping speed of the apparatus, were altered in turn. Then the influence of these control parameters on the particle size was investigated, and the best parameters for the clay in Hefei were identified. The grain size distributions measured by laser method under the best condition and traditional densimeter method were compared, which shows that laser method has a wider measuring range, higher accuracy and better repetitiveness in measuring the particle size distribution of fine grained soil.