A method and a device for modifying a network slice are used for solving the problem that the existing NSI modifying mode cannot meet the end-to-end service requirement. The method comprises the following steps: the method comprises the steps that a first NSSMF receives a first management message sent by an NSMF, the first management message is used for requesting the first NSSMF to modify a first NSSI to be modified, the first management message carries modification requirement information for modifying the first NSSI, and the first NSSMF is an NSSMF for managing the first NSSI; the first NSSMF completes modification of the first NSSI based on the modification requirement information of the first NSSI; and when the first NSSMF determines that the TN part of the transmission network related to the first NSSI needs to be modified, the first NSSMF sends a second management message to a transmission network management system for managing the TN, wherein the second management message carries modification requirement information for modifying the TN part.