At present silkworm cocoons, dried, boiled and reeled, are the female and male mix, so it's very difficult to produce high grade raw silk in large quantity. In the progress of the silk reeling what we can do to separate the complex of female and male silkworms still encounters presently certain difficulties. In this paper the intrinsic relaxation time of the free water and the bound water in silkworms was analyzed and the relaxation model of silkworm was established by use of the proportion difference of the free water and the bound water in the silk gland tissue of the midriff. Finally the gender of the silkworm has been distinguished exactly without destruction by the T2 weighted imaging of magnetic resonance imaging (MRI) technology in the experiment designed. The result of experiment has enormous instruction significance to the production of the silkworm cocoon mixed at the present time.