Embodiments of the present invention disclose a data transmission method, user equipment, and a base station. The method include: receiving a first reference signal set; obtaining, based on the first reference signal set and a first codeword-to-layer mapping, CSI that includes a rank indicator RI and a channel quality indicator CQI, and sending the CSI; receiving a second reference signal set; and receiving, based on the second reference signal set and a second codeword-to-layer mapping, data, where a quantity of layers to which at least one codeword is mapped in the first codeword-to-layer mapping is less than a quantity of layers to which a codeword is mapped in the second codeword-to-layer mapping. According to the embodiments of the present invention, precision of an MCS used during data transmission may be improved, and a throughput of a communications system may be enhanced.