This application describes a data transmission method, a terminal device, and a network device. The method may include determining, by a terminal device, a reporting type of channel state information (CSI), where the reporting type is used to indicate a relationship between CSI of a beam that is currently reported by the terminal device and CSI of a beam that is previously reported by the terminal device in a current reporting period. The method may also include sending, by the terminal device, CSI of N beams to a network device based on a reporting type and a codebook parameter, where the codebook parameter is used to indicate a quantity N of the currently reported beams, and N is an integer greater than or equal to 1. According to the data transmission method, the terminal device, and the network device described in the embodiments of this application, CSI reporting flexibility can be improved, thereby improving system performance.