Abstract: To explore the responses of urban trees to large and continuous nitrogen deposition is one of the foundations for solving the functional issues of urban green space ecosystems. With the advantages of wide adaptability, drought tolerance, low management cost and no thorns, R. pseudoacacia 'Hongsen' can be used as a substitute for poplar trees in urban street trees or shelter forests. In order to investigate the response mechanism of R. pseudoacacia 'Hongsen' to continuous nitrogen (N) deposition and the specificity of its biomass allocation strategy, a 6-month controlled experiment simulating short-term N deposition was conducted with NH₄NO₃ as the N source. Four treatments were established: CK (control, 0 g·m⁻²·a⁻¹), LN (low N, 5g·m⁻²·a⁻¹), MN (medium N, 10 g·m⁻²·a⁻¹), and HN (high N, 15 g·m⁻²·a⁻¹). The results showed that: (1) N deposition significantly increased plant height, ground diameter, specific leaf area (SLA), as well as the biomass of stems, leaves, roots, and total biomass, and significantly decreased specific root length (SRL); (2) SRL, SLA, and plant height exhibited a "low-promotion and high-inhibition" response to N deposition. The biomass under LN and HN treatments was primarily allocated in the aboveground parts, whereas the biomass under MN treatment tended to be allocated in the roots; (3) N deposition increased N content in stems, leaves, and roots of R. pseudoacacia 'Hongsen' seedlings, and its plant height (H), ground diameter (D), specific leaf area (SLA), stem biomass (SB), leaf biomass (LB), root biomass (RB), root mass ratio（RMR）and root-shoot ratio（R/D）were all positively correlated with N content, while SRL and SMR were negatively correlated with plant N content. These findings indicated that R. pseudoacacia 'Hongsen' had distinct biomass allocation strategies to adapt to N deposition, and a certain level of N deposition could promote the productivity of R. pseudoacacia 'Hongsen'.