As an important Chinese medical herbs, how to directive cultivationof Urtica dioica is an important issue both for scientists and farmers. In this paper, the same cuttings of this herbs grown at 5 soil water gradients, 93.58%（CK）,8074%(T1), 67.90%(T2), 55.06%(T3) and 42.22%(T4) were select to determine the effects of soil water content on growth rate, biomass accumulation and allocation, photosynthetic-related parameters and our specific aim was to find the optimum range of soil water for the cultivation of this herb, and their effect on biomass allocation, which may form a base for the man-made cultivation in north China. Following conclusion was found. The growth and biomass increased with the decrease in soil water at begin, then decreased when the soil became too dry. Optimum soil water for the largest biomass and growth rate was 80.74% and a range of soil water from 67.90% to 80.74% could induce a linear increase in biomass accumulation. Similar to the changes in biomass, photosynthetic rate, stomatal conductance, transpiration rate and stomatal limitation showed a first-increase-then-decrease pattern, too and out of that range from 67.9% to 80.74%, photosynthetic capacity sharply decreased and accumulation of dry mass also reduced. Furthermore, soil water directly adjusted the allocation of photosynthesis between above and below ground via root/shoot ratio and plant shape of height and diameter. In the practice of cultivation, soil water control may be an effective measure for the inducement of one specific organ for a special utilization.