According to the method for adding a secondary serving cell, the apparatus, the device, and the network system that are provided in the embodiments of the present invention, a secondary base station sends a secondary serving cell addition response message to a primary base station, so that after the primary base station sends a reconfiguration message to a terminal, it is ensured that the secondary base station can correctly receive a random access request message sent by the terminal, and the secondary base station then can determine, according to the random access request message, content of a random access response message and a time for sending the message to the terminal. According to the embodiments of the present invention, a primary serving cell and a secondary serving cell can still work properly in a case in which the primary serving cell and the secondary serving cell belong to different base stations, or when a delay over a connection between the primary serving cell and the secondary serving cell is relatively long and a capacity is relatively small.