The present invention discloses a data retransmission method, apparatus, and system, and relates to the field of communications. In a data retransmission method, a special codeword is determined according to a difference between a number of layers selected by a transmitter and the number of layers that need to be occupied for retransmitting a codeword. The number of layers that need to be occupied by the codeword to be retransmitted is smaller than the number of layers selected by the transmitter and no new data needs to be transmitted and wherein the number of transport blocks comprised in the special codeword is equal to the difference. The codeword that needs to be retransmitted and the special codeword are allocated to each transport layer through a layer mapping.