Embodiments of the present invention provide a data transmission method for a multi-antenna system, and a device. The method includes: performing first precoding processing on to-be-sent data in a first antenna direction of a multi-antenna system to generate first data; performing second precoding processing on the first data in a second antenna direction of the multi-antenna system to generate second data; and sending the second data to a receive node through each antenna port of the multi-antenna system; wherein: when the first precoding processing is first transmit diversity processing, the second precoding processing is second transmit diversity processing or second transmit spatial multiplexing processing; when the first precoding processing is first transmit spatial multiplexing processing, the second precoding processing is second transmit diversity processing. The method and the device are used to send data by using antennas in multiple antenna directions in the multi-antenna system.