Embodiments of the present disclosure discloses a method and apparatus for precoding a reconfigurable intelligent surface (RIS), the method is performed by a terminal device, including: determining a precoding matrix indicator (PMI) and an additional deflection phase angle for each RIS unit group in an RIS array according to channel information of the each RIS unit group; and sending first indication information, where the first indication information indicates the precoding matrix indicator and the additional deflection phase angle for the each RIS unit group. Therefore, a terminal device indicates, to a network device or an RIS array, a PMI and an additional deflection phase angle which correspond to each RIS unit group, such that the network device or the RIS array performs precoding on each RIS unit group, thereby preventing the problem of a precoding scheme failing due to the RIS array not meeting the far-field hypothesis, and also reducing the complexity of RIS precoding.