Abstract: This paper briefly describes the rule of the settlement of soil grains in water, the experimental principle of mechanical analysis with a hydrometer and the manufacturing and designing method of hydrometer A and B. The requirement in accuracy for mechanical analysis with the hydrometer is presented also. We suggest that the geotechnical laboratories using hydrometers and the manufacturing factories co-operate closely to produce a unified and qualified type of hydrometer for mechanical analysis, So as to eliminate the work of corrections for hydrometer scale in our country and to improve the experimental quality and efficiency of our work.