A method and an apparatus for protecting data carried on an Un interface between a eNB and a relay node are disclosed. Three types of radio bearers (RBs) are defined over the Un interface: signaling radio bearers (SRBs) for carrying control plane signaling data, signaling-data radio bearers (s-DRBs) for carrying control plane signaling date; and data-data radio bearers (d-DRBs) for carrying user plane data. An integrity protection algorithm and an encryption algorithm are negotiated for control plane signaling data on an SRB, control plane signaling data carried on an s-DRB, and user plane data carried on a d-DRB. With the respective integrity protection algorithm and encryption algorithm, the data over the Un interface can be protected respectively. Therefore, the security protection on the Un interface is more comprehensive, and the security protection requirements of data borne over different RBs can be met.