A data transmission method is provided, which includes: obtaining a current queue length of a queue corresponding to an output port; when the current queue length meets a back-pressure requirement, determining a back-pressure priority corresponding to the current queue length according to the current queue length and a mapping relationship between a preset queue length and the back-pressure priority, and generating back-pressure information, where the back-pressure information inhibits a line card from sending data with a data priority less than or equal to the back-pressure priority to the output port; and sending the back-pressure information to a line card.