Chongming Island, which locates at Yangtze River mouth, is the largest estuary alluvial island in the world. Plant configuration mode in riparian banks plays different functions on decreasing soil erosion and bank collapse. Four plant configuration modes located at collapsing banks of south coast of Chongming Island were studied in the paper, the four modes includes Phragmites australis + Scirpus mariqueter mode (PSM), Taxodium ascendens + Phragmites australis + Scirpus mariqueter mode (TAPSM), Salix matsudana×alba+ Phragmites australis + Scirpus mariqueter mode (SPSM) and Taxodium distichum + Phragmites australis + Scirpus mariqueter mode (TDPSM). The spatial distribution of root system in four modes at different beaches were investigated and analyzed. The results showed: (1) The largest mean total root length in 0-40 cm soil layer happened in SPSM, and the value was 137.0 cm/cm2, the smallest value (91.4 cm/cm2) happened in TAPSM. The ratio of herbaceous root length ranged from 94.6% to 98.1% in three arboreal and herbaceous mixed modes. (2) Except SPSM mode, root length density (RLD) of other three plant configuration modes generally decreased along with the increase in depth, and all the three modes had their largest RLD in top 10 cm soil layer, with 151 times(PSM), 4.9 times(TDPSM) and 2.0 times (TAPSM) of the bottom layer, respectively; As to SPSM, the largest RLD happened in the range of 10-20 cm in depth. (3) Very fine roots (diameter Φ＜0.1mm) contributed most to total RLD in all four modes, with the ratio ranging from 74.7% to 81.7%. The next portion were fine roots (diameter 01mm≤Φ＜1mm), and large roots (diameterΦ≥5mm) were rarely found. (4) Rank sum test showed: The differences in RLD were not uniform among four modes in lower, middle and higher beaches. Roots can improve soil anti-erosion capacity. The researches on spatial distribution of root system of four plant modes could provide some scientific basis for construction of riparian plant configuration modes, and protecting vegetation zone for anti-erosion in the bank of Chongming Island.