Embodiments of the present invention provide a method, a device, and a system for sharing transmission bandwidth between different systems, and belong to the communications field. When a first communications system and one or more other communications systems share transmission bandwidth, the method includes: comparing, by a first network element in the first communications system, the sum of rates at which a service is sent to a second network element in the first communications system with bandwidth preconfigured for the second network element; adjusting, by the first network element based on a comparison result, a transmission priority of the service sent to the second network element; and adjusting, by the first network element based on transmission congestion status of the first communications system, sending bandwidth for transmitting the service to the second network element.