Provided are a method and an apparatus for establishing a secondary connection. The method includes: acquiring secondary network access capability information of a terminal device, where the secondary network access capability information is used to indicate a capability for the terminal device to access a secondary access network device; sending a secondary connection addition request message to a first secondary access network device, where the secondary connection addition request message is used to request addition of the first secondary access network device as a network device for transmitting data, and includes the secondary network access capability information; and receiving a confirmation message of the secondary connection addition request message sent by the first secondary access network device, where the confirmation message of the secondary connection addition request message includes first configuration information used to establish the secondary connection between the first secondary access network device and the terminal device.