Reactors containing aquatic worms can reduce waste sludge via biological predation by the worms; however, changes in the heavy metals in such reactors could influence the wastewater treatment. The objective of this study is to investigate the mass balance and the transformation of As, Cd, Cr, Cu, Ni, Pb, and Zn in a wastewater treatment system with aquatic worms. In the aquatic worm reactor, the concentrations of heavy metals in the waste sludge increased remarkably, while the total contents changed only slightly. The concentrations and total contents increased to a lesser extent in aquatic worms, while no significant change in these factors was observed in the effluent. As a result, in the activated sludge reactor integrating aquatic worms, the heavy metals in the sludge accounted for a large portion of the overall content (84.7-97.5%) and the residue of As, Cd, Cr, Cu, Pb, Ni, and Zn in the final sludge increased to 36.0, 60.4, 97.2, 94.8, 98.2, 46.9, and 85.5%, respectively, indicating that their bioavailability was reduced.