Plant negative density dependence is the result of interactions between plants themselves and between plants and the environment. We selected a succession series comprising early successional, mid-successional and late successional stages (Artemisia ordosica, Sophora alopecuroides and Stipa bungeana communities, respectively) in a semi-arid area. We investigated plant density and biomass and determined the nitrogen (N) and phosphorus (P) contents of each plant species in each of 225 quadrats, and we calculated the N and P contents of vegetation using biomass as a weighted coefficient. We found that the total plant density of the A. ordosica community increased with the increase in vegetation N:P ratio while the total plant density of the S. bungeana community decreased with the increase in vegetation N:P ratio, with the latter (late successional stage) community exhibiting negative density dependence. In the communities representing the early successional and mid-successional stages, the vertex coordinate of the quadratic function relationship between plant total density and vegetation N/P ratio was (16.6, 353.3); that is, if the N:P ratio of the vegetation was greater than 16.6, the community was characterized by negative density dependence. The analysis showed that the negative density dependence was due to P limitation. These findings reveal that the vegetation N:P ratio in a semi-arid region is the driver of negative density dependence.