Embodiments of the present invention provide a switch, a controller, a system, and a link quality detection method, and pertain to the field of network technologies. In the present invention, the controller determines a detection path, and controls each switch on the detection path to obtain a detection packet, so that after determining that the obtained detection packet matches a prestored flow table entry, each switch adds OAM information to the successfully matched detection packet; and the controller obtains a termination detection packet, and determines link quality according to the termination detection packet. Each switch on a detection path does not need to use a corresponding link quality detection protocol to detect link quality, and only needs to add OAM information to an obtained detection packet. In this way, not only a structure of each switch can be simplified, but also operations of each switch are simplified.