This paper is to study a theoretical equation used to calculate field moisture capscity instead of complicated measurement in situ. It is considered that this moisture coostant includes two parts, i.e. that held by the attractive force of soil particles and that held by water surface energy. The farmer is calculated by 1.5 times of maximum hygroecopicity(Wm),i.e.% of Fnoisture at wilting point (KWm);the latter is obtained by a simplified spoil model in ideal arrangement of ball particles. It is assumed that the number of soil pores is equal to the number of soil particles.