The control of Monochoria vaginalis (Burm. f.) Presl var. plantaginea (Roxb.) Solms-Laub. has been difficult in organic rice cultivation. Here, we evaluated the ability of rice to suppress M. vaginalis through light competition by measuring the relative photosynthetic photon flux density (R-PPFD) calculated as a ratio of the photosynthetic photon flux density below or above the rice canopy. Field experiments were carried out to compare R-PPFD and M. vaginalis shoot dry weight and capsules production under different plant densities. We observed that regardless of plant density, R-PPFD began to decline from 27 days after rice transplantation and remained nearly constant after the heading stage (after 82 days). Furthermore, R-PPFD decreased with increasing plant density. A positive relationship was observed between R-PPFD and M. vaginalis shoot dry weight. The number of capsules of M. vaginalis decreased with increasing plant density. Our study suggests that increasing the plant density of rice decreased R-PPFD below the rice canopy and inhibited the growth of M. vaginalis.