According to the hypothesized mechanism of biological invasion, which is based on traditional niche theory, niche differentiation between alien species and native species is the key to the success of invasion and the damage it causes. Ageratum conyzoides L. is a malignant invasive species that is spreading rapidly in southern China. In order to understand the ecological relationships between A. conyzoides and native weeds in the community and to explore the possible mechanism of A. conyzoides invasion and its harmfulness, an investigation of the weed community was carried out in southwestern Zhejiang Province (28°06'-28°44'N, 119°32'-120°08'E) from 2014 to 2016. A total of 109 plots (1 m×1 m) and 286 plant species were investigated in the following habitats: roadside, abandoned farmland, streamside, and open forest. For each plot, all aboveground plant material was harvested, classified, and weighed according to species. In addition, slope, aspect, and altitude of the plot as well as the rate of sunshine arriving at the plot were measured. Moreover, pH value and the contents of total nitrogen, alkaline nitrogen, effective phosphorus, available potassium, and organic material in superficial soil (0-10 cm depth) of each plot were tested. Levin's niche breadth, Pianka's niche overlap, probability of n-dimensional niche overlap, and interspecific correlation coefficients of A. conyzoides and 16 common native weeds were calculated, and a canonical correspondence analysis (CCA) was conducted. The results showed that (1) the degree of niche overlap between A. conyzoides and common native weeds was significantly higher than that between any two native weeds, due to the largest niche breadth of A. conyzoides. (2) The Pianka niche overlap index between A. conyzoides and common native weeds was in the middle or lower level of 0.04-0.296. (3) The probabilities of n-dimensional niche overlap of common native weeds with A. conyzoides were significantly higher than those of A. conyzoides with common native weeds. (4) A. conyzoides was distributed in the central region of a CCA ordination chart, which indicated that A. conyzoides was a mesophilic species; moreover, exotic weeds (including A. conyzoides) and native weeds were fully mixed in the ordination chart, which indicated that there was no obvious specialization of exotic species relative to native species; and (5) there were no significantly interspecific negative correlations among the 17 common weed species; only 7 species pairs showed significantly positive correlation, and most species did not correlate with each other. In general, the weed community in the rural areas of southwestern Zhejiang Province is unstable, with relatively sufficient resources and low niche overlap among species. Low competition resistance from native plants and large niche width of A. conyzoides lead to extensive and serious invasion of A. conyzoides in the rural area of southwestern Zhejiang Province.