A quality of service monitoring method, system, and a device, where the quality of service monitoring method includes: obtaining, by a first device, information about a first transmission delay for packet transmission between the first device and a second device for a terminal; sending, by the first device, the information about the first transmission delay to a third device; and sending, by the first device at a first moment, information related to the first moment to the third device, where the information about the first transmission delay and the information related to the first moment are used to determine information about a second transmission delay for packet transmission between the second device and the third device for the terminal.