A data transmission method for a multi-antenna system, comprising: performing a first precoding processing on to-be-sent data in a first direction of an antenna array of a multi-antenna system to generate first data, wherein the first direction of an antenna array comprises at least two antennas; performing a second precoding processing on the first data in a second direction of an antenna array of the multi-antenna system to generate second data, wherein the second direction of an antenna array comprises at least two antennas; and sending the second data to a receive node through each antenna port of the multi-antenna system; wherein the first precoding processing is a first transmit diversity processing, the second precoding processing is a second transmit diversity processing; wherein the performing a second precoding processing on the first data in a second direction of an antenna array of the multi-antenna system to generate second data comprises: mapping the first data to second mapping data at second data layers, wherein a quantity of the second data layers is not greater than a quantity of antennas in the second antenna direction; and performing the second precoding processing on the second mapping data to generate the second data; wherein the mapping the first data to second mapping data at second data layers comprises: mapping the first data to the second mapping data at the second data layers, wherein each of the second data layers comprises same data; and wherein the performing the second precoding processing on the second mapping data to generate the second data comprises: performing the second transmit diversity precoding processing on the second mapping data to generate the second data. performing a first precoding processing on to-be-sent data in a first direction of an antenna array of a multi-antenna system to generate first data, wherein the first direction of an antenna array comprises at least two antennas; performing a second precoding processing on the first data in a second direction of an antenna array of the multi-antenna system to generate second data, wherein the second direction of an antenna array comprises at least two antennas; and sending the second data to a receive node through each antenna port of the multi-antenna system; wherein the first precoding processing is a first transmit diversity processing, the second precoding processing is a second transmit diversity processing; wherein the performing a second precoding processing on the first data in a second direction of an antenna array of the multi-antenna system to generate second data comprises: mapping the first data to second mapping data at second data layers, wherein a quantity of the second data layers is not greater than a quantity of antennas in the second antenna direction; and performing the second precoding processing on the second mapping data to generate the second data; mapping the first data to second mapping data at second data layers, wherein a quantity of the second data layers is not greater than a quantity of antennas in the second antenna direction; and performing the second precoding processing on the second mapping data to generate the second data; wherein the mapping the first data to second mapping data at second data layers comprises: mapping the first data to the second mapping data at the second data layers, wherein each of the second data layers comprises same data; and wherein the performing the second precoding processing on the second mapping data to generate the second data comprises: performing the second transmit diversity precoding processing on the second mapping data to generate the second data. mapping the first data to the second mapping data at the second data layers, wherein each of the second data layers comprises same data; and wherein the performing the second precoding processing on the second mapping data to generate the second data comprises: performing the second transmit diversity precoding processing on the second mapping data to generate the second data.