A method and a device for determining a link state, which are used for solving the problem that the frequency of a controller when sending a route update message in a traditional link monitoring method is too high. In the present solution, a controller sends to a first forwarding device a first route update message including a first route, so that the first forwarding device sends to at least one target forwarding device a third route update message including a third route; upon receipt of the third route update message, a second forwarding device determines a target link from the first forwarding device to the second forwarding device, and sends to the controller, according to a monitored state of the target link, a second route update message including a second route and a target field; and the controller determines the state of the target link according to the target field in the second route update message. As the controller only needs to send the first route update message to the first forwarding device to determine the state of the target link, the method can greatly reduce the frequency of the controller when sending the route update message.