A medium access control (MAC) device and method for optical packet-switched metro wavelength division multiplexing (WDM) slotted-ring networks are used for providing quality of service (QoS) guarantees for isochronous traffic. The device includes a MAC processor and a distributed call admission control (CAC) module. The CAC module in each node of an optical packet-switched network is designed in a distributed manner, and flexibly allocates a reserved bandwidth to an isochronous traffic by a mean-rate-reservation method, controls a quota of the isochronous traffic below a quota ratio rH, resolves output contention by recording node locations forming a connection, and establishes a connection for each isochronous traffic. The MAC processor establishes a connection in the reserved bandwidth for each isochronous traffic between the nodes according to control information carried in a control channel, controls uploading, unloading, and erasing of a plurality of data channels, and updates corresponding contents in the control information.