Abstract: IP (Internet protocol)/DWDM (dense wavelength division multiplexing) optical Internet is one of the mainstream networking techniques as backbone for next generation Internet (NGI), and how to improve its fault tolerance capability and quality of service (QoS) multicast provision becomes critical. In this respect, the fault-tolerant QoS multicast routing is one of the effective solutions. A scheme of preventive fault-tolerant QoS multicast routing is therefore proposed on the basis of ant colony algorithm and dual-tree protection mode. Aiming at minimizing the fault rate of light tree, a principle and standby light tree are set up to meet the requirement of restriction on QoS for fault tolerance. Simulation results show that the proposed scheme is both feasible and effective with better performance.