The concentration of heavy metals (Cd, Cr, Cu, Mn, Ni, Pb, Zn) in roadside tree of Sophora japonica L. and their organs along Taijichang Street was measured by using the Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) to compare the heavy metal enrichment capacity between different diameter classes. The results showed that the concentration of heavy metals in different diameter classes of Sophora japonica L., the concentration of the same element in different organs, and the concentration of different elements in the same organ were all different. In general, heavy metal concentration in Sophora japonica L. followed the order of Zn > Mn > Cu > Pb > Cr > Ni > Cd. The heavy metal absorption capacity in all organs followed the order of bark > root > leaf > branch > stem. Meanwhile, the study showed that the enrichment capacity of Sophora japonica L. in diameter at breast height (DBH) of small size was stronger than those of medium and large size. Heavy metal accumulation in different organs followed the order of root > trunk > branch > leaf, which also increased in Sophora japonica L. with the increase of DBH and biomass. By comparing the accumulation per year, the bioenrichment unit area as well as the bioenrichment unit space of heavy metals with medium and large DBH, Sophora japonica L. of smaller DBH (DBH less than 30 cm or more than 20 cm) had higher enrichment efficiency.