Wetlands are a unique ecosystem formed by the interaction of water and land, and have the function of maintaining different plant community types, and greatly promote species richness of the region. Thus, wetlands have become a hot research area for biodiversity conservation. Wetlands are important for water and carbon cycle regulation, wildlife survival, biological diversity, and economic value; however, human activities along with severe environment changes have led to critical loss and degradation of wetland ecosystems. In particular, soil salinity accumulation exerts a significant impact on wetland plant diversity. In previous studies, soil salinity has been considered an important factor affecting plant diversity in arid and semiarid areas. This study aimed to characterize the relationship between plant diversity and other soil factors along the soil salinity gradient in the middle reaches of the Heihe River. There were 27 sites along the middle reaches of the Heihe River and a total of 87 sampling plots were investigated from July to August 2017. These included 9 sampling plots for trees and shrubs (30 m×30 m) and 78 sampling plots for herbs (1 m×1 m). Soil characteristics were analyzed, including soil salinity (SA), soil pH (pH), water content (WC), bulk density (BD), soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), available nitrogen (AN), available phosphorus (AP), and available potassium (AK). The sampling plots were divided into three salinity gradients by cluster analysis and included low salinity (S1), middle salinity (S2), and high salinity (S3) gradients. The results showed that a total of 102 plant species occurred in the plots, belonging to 30 families and 71 genera, indicating that the plant composition was relatively rich. Along the increasing soil salinity gradient, the community composition changed significantly, and the number of families, genera, and species decreased. The Margalef richness index (R) decreased from 1.03 to 0.55 and the Shannon-Weiner diversity index (H) decreased from 1.45 to 1.08, indicating that plant diversity decreased with the increasing gradient in the study area. The effects of other soil factors on plant diversity under different salinity gradients were studied using a redundancy analysis (RDA). The results showed that there were differences in soil factors affecting plant diversity under different salinity gradients. In low salinity pH, AK, and TN, in middle salinity pH, AP, and AK, and in high salinity SOM, TP, AK, and AN affected plant diversity. These findings are of great significance to the maintenance and management of plant diversity in the middle reaches of the Heihe River.