Free swelling ratio is a significant index for classification of expansive soil and identification of swelling potential. Unfortunately it is influenced by many factors

among which the effect of soil sample preparation is discussed. Test researches show that soil sampling mass and test results is of little influence by three funnel bores(4.5

5

5.5 mm) and three drop heights (5

10

15 mm) while they are much influenced by degree of grinding of soil. The specified standard that soil particle size is less than 0.5 mm is not perfect and the results of free swelling ratio for three soil particle sizes(0.25－0.5

0.1－0.25

<0.1 mm) is widely variable

it is discovered that the finest soil(<0.1 mm) is reduced by 30 %－45 % and isn’t consistent with other indexes for expansive soil classification. The contrast is studied and mass difference of particle up to 2 g measured by a cup of 10ml is found; the conclusion that the results is influenced by particle mass is established and the changing of structure is a possible reason. At last the particle size between 0.1 mm and 0.5 mm is suggested.