The species-area relationship is one of the fundamental subjects in community ecology research and an important way to understand plant community structure. Different sampling methods have significantly differing effects on the species-area relationship. In order to realize the minimum area of riparian community in the mountainous areas of Beijing that should be sampled to determine the species richness, we employed a method where the sampling area was gradually expanded. Surveys of the riparian sites were undertaken and the minimum sampling area required for different types of riparian habitats along the Huaijiu River was calculated. The results show that there were 255 plant species, belonging to 70 families of 185 genera. By using clustering analysis, we divided the Huaijiu riparian habitat into six typical classes, including natural riparian, near-natural riparian, artificial bank plant riparian, artificial bank ornamental plant riparian, artificial bank sparse plant dry-stone riparian, and artificial bank masonry riparian. According to the Akaike Information Criterion, the best estimation model for natural riparian, near-natural riparian, artificial bank plant riparian, and artificial bank sparse plant dry-stone riparian is S=c-ae^-bA, for artificial bank ornamental plant riparian this is S=aA/(1+bA), and for artificial bank masonry riparian this is S=c/(1+ae^-bA), whereA is sample area, S is the species number in A, a, b, c is parameters to be estimated and e is the base of the natural log. To meet the criterion that the same proportion of plant species are surveyed, it is obvious that there exist significant differences for different types of riparian habitat type. For the criterion that 80% of the plant species in a riparian habitat is to be surveyed, a sampling area of 84m² was required for artificial bank masonry riparian, which was the smallest. This was 217m² for natural riparian, followed by the artificial bank sparse plant dry-stone riparian (362m²), near-natural riparian (450m²), and artificial bank plant riparian (460m²). The largest area of 571m² was required for artificial bank ornamental plant riparian. These results are very important for conservation of plant diversity in riparian habitats, as they provide a guide to minimum areas required for protection and ecological restoration.