Plant negative density dependence is the result of interactions between plants and between plants and the environment. We selected a succession series, i.e., early successional, mid-successional and late successional stages for Artemisia ordosica, Sophora alopecuroides and Stipa bungeana communities, respectively, in a semi-arid area. We investigated the density and biomass and determined the nitrogen (N) and phosphorus (P) content of every plant species for each quadrat of 225 quadrats, and calculated the N and P content of vegetation using biomass as a weighted coefficient. The results show that, total plant density of the A. ordosica community increased with the increase of vegetation N:P ratio, while total plant density of the S. bungeana community decreased with the increase of vegetation N:P ratio, which took on negative density dependence at the late successional stage. In the early and mid-successional stages of the community succession, the stagnation point of the quadratic function relationship between plant total density and vegetation N/P ratio was (16.6, 353.3), that was, if the N:P ratio of the vegetation was greater than 16.6, which was characterized by negative density dependence. The analysis shows that the negative density dependence is due to P limitation. These findings reveal that the vegetation N:P ratio in a semi-arid region is the driving force for negative density dependence.