A method and device for processing data are provided. The method comprises: a terminal device controlling the current state of a copied data transfer function according to first indication information, wherein the first indication information is used for indicating whether the copied data transfer function is enabled; and the terminal device processing, according to the current state of the copy data transfer function, each RLC service data unit (SDU) in a first radio link control (RLC) cache and a second RLC cache, wherein the first RLC cache corresponds to a first RLC entity, and the second RLC cache corresponds to a second RLC entity.