Example embodiments of this application provide a method and an apparatus for transmitting a service flow based on FlexE. The method includes: sending, by a first network device, a first FlexE overhead frame to a second network device, where the first network device and the second network device transmit a service flow using a first FlexE group, and the first FlexE overhead frame includes FlexE group adjustment identification information and PHY information of a physical layer (PHY) included in a second FlexE group; receiving, by the first network device, a second FlexE overhead frame sent by the second network device, where the second FlexE overhead frame includes FlexE group adjustment acknowledgment identification information; adjusting, by the first network device, the first FlexE group to the second FlexE group; and sending, by the first network device, the service flow to the second network device based on the second FlexE group, to dynamically adjust a FlexE group.