Abstract: Canna indica growing in biological grid were analyzed for its growth and surface area distribution characteristics of its root system.A geometric calculation method was designed to calculate surface area of its root system by determining diameters of its second-class and third-class roots,separately.The result of the calculation by this method is 10 times as accurate as those by the common method,in which calculation is done based on root diameters regardless of root classes.The surface area of the canna roots is 39.4 m²,and the surface area of the third-class roots is 22.2 times as big as the second-class roots in the system.In the processes of eutrophication waterbody treatment by biological grid system the third-class roots play an important role in absorbing N and P,carrying microorganisms,enhancing adhering capacity of plant with fillers and improving the spatial structure of root system and filler

Key words: surface area of root systern, Canna indica, biological grid, third-class root