Reaumuria soongorica and Nitraria tangutorum are two dominant species of the arid region of Northwest China. The paper is to explore the responses of R.soongorica and N.tangutorum seedlings to changed precipitation patterns,to get an insight into the evolution direction and evolution rate of the desert vegetation in Northwest China under future global climate change. A simulated experiment was conducted in 2014 to examine the effects of precipitation quantity and interval on two desert plant seedlings at the Linze Inland River Basin Research Station of Chinese Academy of Sciences. The results show as follows:(1)Compared with the controlling group,there were no significant changes in two desert plants taproot vertical depth when the total precipitation quantity was increased by 30%(P>0.05),but the plant height and basal diameter of the two desert plant seedlings significantly increased,and even significant(P<0.05)when precipitation interval was extended at the same time. The plant height and basal diameter of R.soongorica seedling increased by 25% and 29% and N.tangutorum seedling increased by 10% and 48% when the precipitation quantity was increased by 30%,respectively. When total precipitation quantity was decreased by 30%,though the plant height and basal diameter of tow desert seedlings had no significant changes,the taproot vertical depth of two desert seedlings on average increased by 21% and 40%,respectively,and the increase was even more significant(P<0.05)when the precipitation interval was extended at the same time.(2)Aboveground biomass and underground biomass of the two desert plant reached maximum when the precipitation interval was extended and the total precipitation quantity was increased by 30% at the meantime,but their root-shoot ratio were relatively small. When the total precipitation quantity was decreased by 30% and the precipitation interval was shortened at the meantime,the two kinds of desert plants tended to allocate more biomass to the underground,and the root-shoot ratio reached maximum which were 0.64 and 0.78 for Reaumuria soongorica and Nitraria tangutorum,respectively. Therefore,the growth of dominant desert plants seedlings were mainly affected by the total precipitation in arid desert ecosystem,but the promotion effects of increasing precipitation depended on precipitation interval. It is beneficial to the growth of two desert plants by extending the precipitation interval and increasing the precipitation quantity by 30% at the same time. The growth of two desert seedlings were restrained when the precipitation quantity was decreased by 30%. The biomass allocation pattern of two desert seedlings were changed by root elongation growth to adapt to drought stress.