Embodiments of the present invention disclose a method, an apparatus, and a system for data transmission. The method for data transmission includes: determining that data to be transmitted is control plane signaling related to a user equipment that camps on a relay node; and transmitting the data through a first user data bearer established between the relay node and a donor base station, where the first user data bearer provides integrity protection for the data. According to the embodiments of the present invention, when the control plane signaling related to the user equipment that camps on the relay node is transmitted between the relay node and the donor base station, integrity protection is provided for the control plane signaling, and therefore attacks such as the denial of a service attack are prevented.