We studied changes in biomass and components of one-year-old Myricaria laxiflora
seedlings under different simulated soil water levels to reveal the response of seedling growth to these changing conditions. Results showed that the growth characteristics (aboveground and underground biomass, plant height, and length of main root) of one-year-old seedlings increased at first and then decreased with decreased soil water level. The highest aboveground and underground biomass values reached 0.0438 g and 0.0100 g, respectively, and the lowest values were 0.0177 g and 0.0026 g, respectively. The highest aboveground biomass was observed in the -10 cm treatment, whereas the highest underground biomass was observed in the -15 cm treatment. The seedling component indicators, including seedling diameter, root surface area, plant height, root volume, primary branch number, and secondary branch number, also reached their highest values in the -10 cm or -15 cm treatments. There was a significant correlation between the growth of the main components of the seedlings and the change in soil water level. Principal component analysis indicated that the underground part of the seedlings was susceptible to changes in soil water level, and that the trait syndromes of seedling growth changed with changing soil water level. This suggests that the investment strategies of seedling growth differed among the different soil water levels.