Yuanjiang dry-hot valley is poorly vegetated as a fragile arid ecosystem. Although it was lushly forested in the past, it becomes a tropical montane savannah in recent decades. This study focus on the regime driving such a change which could help to understand the relationship between plants and their living environments in the low land of dry-hot valley, and also help to thrive the ecology policy in Yuanjiang-Red River. Plant species and their individuals are investigated in transects and plots arranging along the river channel. The first is L-transect lying along the river running direction while the second is P-transect lying perpendicular to river channel. Alpha and beta indices are employed to cope with biodiversity change in species and in environments. Expected species, rarity and abundance indices are subscribed to the correlation among species, population size, and their living circumstances, in the terms of Species_estimated, Singeltons, Uniques, ACE, ICE, and Chao2. Meanwhile, fifty years’ meteorological records in the valley including temperature and precipitation are collected for environmental references. The results indicate that: (1) in Yuanjiang dry-hot valley, alpha diversity is higher in the transects and plots closing to river channel than apart from while beta diversity increases from upper stream to the lower with Bray-Curtis < 0.500, but this condition is reversed in the transect perpendicular to the river; (2) tidal range contributes a lot in alpha biodiversity especially in the upper stream; (3) expected species, rarity and abundance indices are stronger in the sites closing to river channel and tidal range which is strongly indicating heterogeneous habitats. (4) Solanum virginianum is an ideal environmental indicator in the dry-hot valley. Conclusively, plants are not flourished in the low altitude ascribed to high temperature and poor precipitation. More attention should be laid on the micro habitat protection in our environmental management.