A method for transmitting an application programming interface API request includes receiving, by a first API gateway, a first API request; obtaining, by the first API gateway, a first forwarding label corresponding to the first API request, where the first forwarding label includes a first target security domain identifier, and a security domain identifier of the first API gateway is different from the first target security domain identifier. The method also includes determining an address of a second API gateway according to a mapping relationship between the first target security domain identifier and the address of the second API gateway. The method additionally includes sending the first API request to the second API gateway based on the address which is a next-hop API gateway of the first API gateway that sends the first API request to an API gateway corresponding to the first target security domain identifier.