The outdoor experiment was conducted to investigate the effects of snail Parafossarulus striatulus (Benson) on the growth of Vallisneria spiralis, with three treatment groups (0, 150, 450 ind/m²). The results showed that the snails were beneficial for V. spiralis growth. Both of the relative growth rate and plant height in the low density snail group (150 ind/m²) increased by 20% in comparison with the control group with no snails. The high density group improved further, compared to the control group, with increases in relative growth rate, number of leaves, and plant height of 28%, 15% and 27%, respectively. It is suggested that the snails may reduce the periphyton biomass on the leaves of the plant by grazing, contributing to the increased plant growth. This study is helpful for us to further understand the mutualistic relationship between freshwater snails and aquatic macrophytes, as well as the ecological relationships between organisms in aquatic ecosystems.