The invention belongs to the technical field of drilling, and relates to a torque measurement system and method, in particular to a downhole torque measurement while drilling system and method. The downhole torque measurement while drilling system comprises a drill collar body, and torque measurement sensors; a plurality of torque sensor mounting grooves are coaxially formed in the outer wall of the drill collar body at equal intervals, and through holes are formed between the adjacent torque sensor mounting grooves; and each torque measurement sensor is of an elastic circular ring structure mounted in the corresponding torque sensor mounting groove, and a plurality of torque strain measurement bodies which are arranged at equal intervals and share the same circumference are arranged on the torque measurement sensors. Therefore, the downhole torque measurement while drilling system and method have the following advantages that 1, the measurement while drilling mode is adopted, torque borne by a drilling tool can be obtained in real time, and thus supporting is provided for quick adjustment of drilling tool parameters; and 2, the measurement bodies on the circular ring structures ofthe multiple torque measurement sensors are adopted to monitor the torque change, and the measurement accuracy is improved.