Embodiments of the present invention provide an uplink data transmission method, a timer configuration method, and a related device, where the uplink data transmission method is applied to a user equipment, and includes starting a timer when a first operation is performed on first data on a first transmission branch, and performing a third operation on second data when a second operation is not performed on the second data on a second transmission branch as timing of the tinier ends. Preprocessed data is allocated to two transmission branches by the user equipment.