We analyzed the role of niche usage flexibility (i.e. niche width) in promoting species coexistence in competitive communities in a one-dimensional niche space. We included two types of stochasticity, namely, a random sampling effect of community founding and environmental fluctuation. Fluctuation was further divided into two categories: niche-independent fluctuation (synchronized over the niche space) and niche-dependent fluctuation (variable among individual niche positions). In the analysis, two types of genetic and inheritance systems of individual niche position were considered, i.e. sexual reproduction with multiple loci and asexual reproduction with phenotypic plasticity. We found that niche usage flexibility promoted species diversity only under restricted situations when the environment was constant, but it generally promoted diversity when the environment fluctuated. In particular, under niche-independent fluctuation, niche usage flexibility significantly enhanced species diversity. In contrast, the analysis also predicted that when niche flexibility was constant, species diversity decreased with increasing environmental correlation between neighboring niches.