A data transmission method, a terminal and a network side device are provided. The data transmission method includes: receiving, by a terminal, a first message, where the first message includes first transmission configuration indicator (TCI) state indication information; determining, by the terminal, K TCI states of K transmission layers indicated by the first TCI state indication information, according to a first correspondence relationship combination included in pre-acquired TCI configuration information, where the TCI configuration information at least includes a correspondence relationship combination configured to indicate correspondence relationships between TCI states of a plurality of transmission layers and the first TCI state indication information, the first correspondence relationship combination is configured to indicate the correspondence relationship between the TCI states of the K transmission layers and the first TCI state indication information, K is a quantity of layers used for data transmission; performing, by the terminal, data transmissions in the K transmission layers according to the K TCI states respectively.