A data transmission method, device, and system to resolve a problem that encoding cannot be performed based on an incomplete subframe because a length of the incomplete subframe is unknown. A first device generates a first subframe and a second subframe according to first data, determines a redundancy version (RV) of the first subframe according to an RV of the second subframe, and sends M of N orthogonal frequency division multiplexing (OFDM) symbols of the first subframe and N OFDM symbols of the second subframe to a second device.