Abstract: Competition between aquatic plants plays a crucial role in ecosystem recovery in water bodies. Replacement series experiments were performed to explore the interspecific competition between Myriophyllum spicatum L. and Vallisneria natans (Lour.) Hara at different growth stages. Results showed that competitive capacity of M. spicatum L. was superior to that of V. natans (Lour.) Hara whether it was in the seedling or adult stage. Comparatively speaking, adult stage V. natans (Lour.) Hara had an advantage over its seedling stage. The average height of M. spicatum L. grown with V. natans (Lour.) Hara in the adult stage was 128.6% (P< 0.05) higher than that grown with V. natans (Lour.) Hara in the seedling stage, whereas the average dry weight of M. spicatum L. was 62.8% (P< 0.05) lower. The M. spicatum L. grown with V. natans (Lour.) Hara in the seedling stage had more branches and larger plant plexus, thus forming a larger canopy and restraining light use by V. natans (Lour.) Hara seedlings. In contrast, M. spicatum L. grown with adult V. natans (Lour.) Hara had fewer branches, longer plants, and its top floated on the water, which was obviously inhibited due to increased competition from V. natans (Lour.) Hara, though this was less than the intraspecific competition of M. spicatum L. In addition, the average root length of seedling stage V. natans (Lour.) Hara was 28.6% (P< 0.05) longer than that of adult stage V. natans (Lour.) Hara in competition with M. spicatum L., which tended to compete for resources underground. This study showed that the competitive capacity of M. spicatum L. was superior to that of V. natans (Lour.) Hara, and seedling stage V. natans (Lour.) Hara was significantly impacted by competition with M. spicatum L.