A method and apparatus for transmitting a service flow based on a flexible Ethernet, where a bandwidth resource corresponding to a bundling group (BG) of a flexible Ethernet is divided into M timeslots, service data of a service flow is encapsulated in N timeslots in the M timeslots, and the method includes: when a first PHY in the BG fails, determining, based on a preconfigured first timeslot configuration table (TCT), a target timeslot (TTS) in the N timeslots that is mapped to the first PHY; searching the M timeslots for an idle timeslot (ITS) based on the first TCT; adjusting the first TCT when a quantity of ITSs is greater than or equal to a quantity of TTSs, so that all the N timeslots are mapped to PHYs other than the first PHY; and transmitting the service flow by using the mapped PHYs of the bundling group.