Rainfall affects the plant growth. Effects of changing rainfall patterns on the root and biomass of Reaumuria soongorica seedlings were researched in this paper .Seedlings of typical natural growth R. soongorica community as the research object,natural rainfall as check,we set up a experiment with two variables: rainfall quantity and rainfall time interval .The three variation of rainfall quantity were -30% (W^-),natural rainfall (W) and +30% (W⁺ ) .The two rainfall time interval were natural rainfall time interval (T) and 1.5 times of natural rainfall time interval (T⁺). The results showed that: （1） In the condition of same rainfall quantity, the extended rainfall time interval promoted the root length, root surface area and root shoot ratio, which could gain underground water to increase seedling growth. （2）In the condition of W^- and W, all part of biomass would be accumulated by the extended rainfall time interval, but it showed opposite in the W⁺. （3） In the condition of natural rainfall time interval, the main root length and biomass increased markedly, When rainfall quantity was promoted, R. soongorica seedlings growth was advanced at the same time. Visible changes in rainfall patterns will significantly affect seedling growth and biomass allocationin the natural growth R. soongorica community.