A previous study proposed a downlink scheduling of real time variable rate (RT-VR), non real time variable rate (NRT-VR), and best effort (BE) traffic forwarding classes [1] to support QoS of the WiMax network. However, the study did not consider scheduling extended real time variable rate (ERT-VR) service, which defines the additional requirement of jitter performance when compared with RT-VR service. This article studies the supplementing function in ERT-VR service to complete QoS scheduling of downlink traffic in the WiMax network. Simulation results indicate that the proposed scheme not only satisfies delay time, but also guarantees the jitter requirement of ERT-VR traffic.