A method and an apparatus for improving network security. The method includes obtaining, by a control node, alarm information, where the alarm information includes address information of an attack source that attacks a subnet of at least two subnets and identification information of the attacked subnet of the at least two subnets, using, by the control node, the alarm information to sort the attack sources in descending order of threat levels, and using a sorting result as a blacklist, and sending, by the control node, the obtained blacklist to at least one subnet that is not attacked yet in the network system. The method and apparatus are applicable to collaborative defense among multiple subnets.