A pot experiment was carried out to study the growth,N nutrition,and physiological characteristics of Calligonum caput-medusae seedlings under different irrigation treatments (30%,40%,50%,60%,and 85% field water capacity (FWC)),and further to find out the suitable water condition to improve the plant growth.The results showed that:(1)Dry weight of the assimilating branches significantly increased with increasing amounts of irrigation.However,C.caput-medusae seedlings were infected with powdery mildew at the high irrigation level (85% FWC).(2)During the early growth stage,the assimilating branches,as growth center,accounted for 39.5% of the average whole-plant dry matter accumulation.Irrigation significantly promoted dry matter allocation to assimilating branches.During the late growth stage,the plants allocated less dry matter into assimilating branches,accounting for an average of 12.3% of whole-plant dry matter accumulation.In addition,the allocation proportion was not correlated with irrigation amounts.(3)The assimilating branches had high N and nitrate-N concentrations throughout the whole growth season.The N concentration of assimilating branches decreased,but the nitrate-N concentration had a fluctuation change with increasing amounts of irrigation.(4)The effects of irrigation on physiological characteristics of C.caput-medusae seedlings mainly occurred in the early growth stage.The nitrate reductase activity and chlorophyll content of assimilating branches were significantly limited under the low and high irrigation treatments (30% and 85% FWC).The plants had great MDA,soluble sugar and proline contents,but lower water potential under the lowest irrigation treatment.The MDA and proline contents decreased,but water potential and soluble sugar content increased with increasing amounts of irrigation.This suggested that C.caput-medusae seedlings had strong osmotic adjustment capacity to adapt to water stress.However,medium irrigation levels (50%～60% FWC) were recommended for establishing C.caput-medusae seedlings in this area due to plant growth,N assimilation and chlorophyll accumulation could be limited by over-irrigation and water stress.