Abstract: The diversity of understory plants and the composition of Eucalyptus robusta Smith have a significant impact on regional ecosystem functions. However, the effect of introducing Eucalyptus species on understory plant diversity has been a subject of controversy. Similarly, the impact of Eucalyptus species on the quality of regional habitats remains uncertain. This study focuses on Lancang County, Yunnan Province, to investigate the influence of mature Eucalyptus plantations on understory plant diversity using spatial analysis and typical sampling methods. The study adjusts InVEST model parameters based on species diversity indices and NDVI to elucidate the habitat quality characteristics of mature Eucalyptus plantations. The analysis yields three main findings. Following the replacement of evergreen broad-leaved forests, Simao pine forests, and shrub forests by Eucalyptus, both the Sorensen index and the Shannon-Wiener index showed a decrease, indicating reduced diversity in the understory plant community of mature Eucalyptus plantations. Woodland habitats in large areas exhibited higher quality, whereas the habitat quality of mature Eucalyptus plantations was notably lower, especially in the northern and western regions. By verifying the biodiversity security coefficient, the InVEST model, after adjusting habitat suitability parameters, produced more scientifically and reasonably estimated habitat quality.