Nitrogen is an essential element for plant growth and is closely related to plant life activities. In most terrestrial ecosystems, plant growth is limited by nitrogen availability. Therefore, it is important to improve plant productivity in terrestrial systems by increasing the nitrogen uptake capacity of plants. Biochar is regarded as a kind of "soil conditioner" produced by pyrolysis of biomass under high temperature and hypoxia conditions. It has attracted more and more attention because of its important role in promoting crop yield, reducing greenhouse gas emissions and environmental pollution control in recent years. Most previous studies have applied biochar in dryland ecosystems, but few in wetland ecosystem. Though biochar has been proved to be beneficial of plant growth and nitrogen uptake, but whether it is influenced by different nitrogen availability and water conditions is still unclear. Therefore, we conducted a greenhouse experiment to investigate the effects of biochar addition on the growth and nitrogen absorption of Phragmites australis with three nitrogen additions (no addition, 30 kg hm^-2 a^-1, 60 kg hm^-2 a^-1) and two water treatments (inundation and no inundation). The results showed that:(1) biochar addition promoted the root growth of P. australis, increased the root biomass by 40.5% in the non-flooded treatment and by 20.1% in the flooded treatment, but have no significant effects on biomass of other parts and total biomass. (2) Biochar addition could improve the nitrogen absorption rate in the non-flooded condition, increased the nitrogen absorption rate by 23% in average. Biochar addition also enhanced the nitrogen productivity of P. australis in the flooded condition by 22% in average. (3) Biochar significantly decreased the soil ammonium-N, nitrate-N and mineral nitrogen concentrations, increased the soil nitrogen loss, and the effects were the strongest in the non-flooded and high nitrogen treatments because biochar promoted nitrogen absorption rate of P. australis. There was a significantly positive correlation between nitrogen absorption rate of P. australis and soil nitrogen loss. Our study shows that biochar has different effects on growth, nitrogen absorption and utilization of P. australis in different soil nitrogen availability and water treatments, and the difference may be related to the absorption preference of plants on different nitrogen chemical forms. Therefore, it is necessary to comprehensive balance soil water status, nitrogen enrichment and nitrogen absorption preference of plants when applying biochar in field. This result can practically provide scientific support for the application of biochar in the wetland ecosystem.