Plant invasion succeeds because of such invader characteristics as fecundity and high-efficiency multiple reproduction modes. The acquisition of individual resources and the benefits of individual fitness are driven by optimum patterns of life history and trade-offs of reproductive allocation (RA) in plants, and variations in RA strategy play an key role in plant adaptation to environmental changes. Thus determining the RA strategy of invasive plants is important for understanding the successful mechanism underlying plant invasion. This study aims to determine the reproductive allocation (RA) strategy of two herbaceous invasive plants (Conyza canadensis and Solidago canadensis) across different cover classes in eastern China. Plant height, maximum branch length, the reproductive branch number, aboveground biomass, the amount of reproductive investment, and the coefficient of RA of the two plants decreased with increasing cover class (although the changes were not pronounced). Thus the two plants may decrease physiological investment on reproductive behavior and reduce RA under competitive conditions because of interspecies competition that progressively decreased and intraspecific competition that gradually increased with increasing cover class. The RA of the two plants may be principally influenced by plant community composition at low cover classes, but by soil pH at high cover classes. This may be the outcome that soil pH of the two plants decreased with increasing cover class significantly. Thus, intraspecific competition for resources may play an important role in the RA strategy of the two plants under high cover class.