Clarifying the effects of invasive plant species (IPS) on the taxonomic and functional diversity of native communities provides insight into the mechanisms underlying successful plant invasion. This study aims to determine the effects of an IPS ( Solidago canadensis L.) at different invasion degree on the taxonomic and functional diversity of plant communities by using a comparative analysis. The degree of S. canadensis invasion was divided into several categories–uninvaded (0%), low (<35%), moderate (35%–75%), and heavy (>75%)–on the basis of the relative abundance of the species in the invaded ecosystems. Forty quadrats (2 m × 2 m) were surveyed per invasion to assess the degree of S. canadensis invasion. The taxonomic diversity of plant communities was calculated using the Shannon–Wiener diversity and Margalef’s richness indices. The functional diversity of plant communities was estimated using community-weighted mean trait values, Mason’s α and β functional diversity, and Rao’s quadratic entropy. The taxonomic diversity indices of plant communities significantly decreased under moderate and heavy degree of S. canadensis invasion conditions. Most of the functional diversity indices of plant communities significantly increased under all degree of S. canadensis invasion conditions (especially under moderate and heavy degree of invasion conditions). The increased functional diversity of plant communities under S. canadensis invasion conditions (especially under moderate and heavy degree of invasion conditions) may lead to a more efficient use of resources at the interspecific level via niche complementarity.