The embodiment of the invention relates to a method, a device and a system for transmitting user plane data. The method comprises the following steps of: receiving a connection establishment request message sent by a terminal; establishing signaling connection with the terminal according to the connection establishment request message; and transmitting a user plane Internet protocol (IP) data packet through the signaling connection. In the embodiment of the invention, after the signaling connection between user equipment (UE) which is in an idle state and a network side, namely, mobile management equipment is established, an uplink user plane IP data packet or a downlink user plane IP data packet carrying business content is directly transmitted between the UE and the network side through the signaling connection without specially establishing (recovering) a null side user plane bearing, namely, a radio access bearer (RAB) between the UE and the network side, namely, S-GW. Signaling overhead can be saved, so that network load is lightened and the operation cost of an operator is lowered.