The present disclosure relates to flexible-Ethernet data processing methods and devices. One example method includes acquiring a to-be-switched first client service flow, where the first client service flow is a service flow suitable for transmission on a flexible Ethernet, performing first rate adaptation from a source clock domain to a target clock domain on the first client service flow to obtain a second client service flow that matches the target clock domain, and performing serial-to-parallel conversion on the second client service flow in the target clock domain to obtain a parallel client slot flow.