The present invention relates to a system, a device, and a method for customizing a user-defined network. The system includes a gateway and a mobile network element. The gateway is configured to receive a user-defined network request, request, according to the user-defined network request, an invocation of a communications service corresponding to the mobile network element to establish or modify the user-defined network, and return an invocation result. The user-defined network request includes definition information of an MBB communications service resource required by the MBB application service. The mobile network element is configured to implement invocation of the corresponding communications service. Therefore, the MBB communications service resources required by an MBB application are customized as required, and an invocation of the MBB communications service resources is associated with a requirement of the MBB application, thereby implementing reasonable utilization of the MBB communications service resources.