A data transmission method, device, and system includes, when a preset condition is met, determining a quantity N of first symbols included in one first symbol group and a quantity K of bits transmitted in the first symbol group, where the preset condition is └ log2 mN┘>N·└ log2 m┘, K=└ log2 mN┘, m≥2, N≥2, and K≥3; obtaining X data blocks, where each of the first X−1 data blocks includes K bits, the last data block includes Y bits, X≥1, and K≥Y≥1; and processing each of the X data blocks into N first symbols to obtain NX first symbols, and sending the NX first symbols.