We conducted an experiment on 9 years of continuous nitrogen application to explore the effect of nitrogen addition on plant community structure in Stipa baicalensis steppe, with different nitrogen addition levels of 0(N0), 15(N15), 30(N30), 50(N50), 100(N100), 150(N150), 200(N200) and 300(N300) kg·hm^-2·a^-1. The responses of community structure and important value, diversity as well as biomass to different nitrogen addition in 9 years of continuous nitrogen application were explored. The results showed that: (1) community structure was significantly responded to longterm nitrogen addition with the lowest species number in high nitrogen application rate, while the highest important value in dominant species under high nitrogen application rate was observed. (2) Plant diversity index in S. baicalensis steppe was reduced with varying degrees in longterm nitrogen addition. The dominance index was increased with longterm nitrogen addition with the most prominent value in N300 level addition (P<0.05). (3) The productivity of grassland vegetation (aboveground and belowground biomass) increased firstly and then decreased after longterm nitrogen addition. The peak value of aboveground biomass was observed under N100 level addition, while the maximum value of belowground biomass was appeared at N50 level (P<0.05). Root was characterized at 0-10 cm distribution. (4) RDA analysis showed that aboveground biomass of Carex duriuscula and Bupleurum scorzonerifolium were significantly positively correlated with soil pH (P<0.05), and Thalictrum sguarrosum and Potentilla bifurca were significantly positively correlated with soil organic matter (P<0.05). The community structure in S. baicalensis steppe was greatly responded to longterm nitrogen addition composition with diversity decreased and changes in community composition.