A successional Artemisia ordosica and A. frigid community, located on the eastern edge of the Tengri Desert in Alashan Left Banner, Inner Mongolia, was selected as an object for study. Densities of the plants and total nitrogen (N) and available phosphorus (P) in 0~20cm soil layer were measured of the 400 quadrates (4m×4m) in the sampling plot (80m×80m) for analysis of relationship between N/P ratio in the soil and plant density thereupon and between the two in spatial heterogeneity. Results show that with increasing N/P ratio, the density of A. ordosica decreased, but the density of A. frigid increased; while with intensifying spatial heterogeneity of N/P ratio, the spatial heterogeneity of A. ordosica density weakened but the spatial heterogeneity of A. frigid density displayed no relationship, which suggests that the plant density is closely related to N/P ratio, and A. frigid is displacing A. ordosica as N/P ratio increases. Thus, change in N/P ratio in the soil is one of the causes of the succession of A. ordosica community in the arid zone.