In this study, the 3-year young lateral cyperusorientalis in Beijing area was selected as the object of study. A greenhouse potted experiment was used to set five different soil water content levels, and ¹⁵N isotope natural abundance method was used to study the physiological activities, biomass allocation and nitrogen absorption and distribution of lateral cyperusorientalis seedlings under different water conditions. Results showed that: (1) Under water stress(D1), the individual net photosynthetic rate, transpiration rate and respiratory rate, were low, and increased by 4.86 times, 3.74 times, 7.29 times after relieving drought stress. The leaf water content and leaf area increased by 24.37% and 23.69%, respectively. After that, when the soil water content exceeded the normal value, with the further increase of soil water content, all indexes decreased slightly. (2) Under water stress, biomass distribution and nitrogen distribution were inhibited. With the increase of water content, biomass distribution was shown as the aboveground part > underground part. The nitrogen distribution rate was shown as follows: root > leaf > stem. The nitrogen in root and leaf were transferred to the stem with the increase of soil water content. (3) When the soil water content reached the 70% to 80% of field water capacity, the growth and development of platycladusorientalis seedlings showed the best.