The present disclosure provides a method, an apparatus, and a system for cooperative defense on a network. Alarm information sent by a security device of a first subnet that is being attacked is received by a controller; the controller generates flow table information according to the alarm information, and forwards the flow table information to a switching device of the first subnet and a switching device of at least one second subnet, which is equivalent to that, after detecting an attack, a security device of a subnet generates alarm information, and shares, by using the controller, the alarm information with a switching device of the subnet and a switching device of another subnet that is not being attacked, to form networkwide cooperative defense, thereby enhancing network security.