Abstract: The use of the formula for calculating the grain diameter in densimeter method prescribed in “Standard for soil test method” GB/T 50123—1999 is limited to the small global grains with low velocity in a layer flow considering only the viscous resistance and effective gravity. The soil grains tested all have irregular shape.The surrounding granules cause additional action to the settling granules (namely, intervenient settling). These factors will reduce the settling velocity of granules greatly. In addition, there will be a turbulent flow in the suspending liquid when the settling velocity of granule is bigger. This will outstep the applicable range of Stokes formula. The above-mentioned factors will cause that the calculated results deviate from reality. This paper dissects the above-mentioned factors,and proposes the corrected Stokes formula and its applicable range.