The invention provides a device and a method for measuring the symmetry of a rudder shaft slot of a rudder shaft part. A gauge block I, a gauge block II and a gauge block III are connected side by side above a base through connecting screws. A dial gauge is adsorbed to the base by magnetic force, and is arranged near the gauge block I, the gauge block II or the gauge block III as needed during measurement. The gauge block I, the gauge block II and the gauge block III are set to adapt to rubber shafts with different slot width, and can be disassembled. During measurement, one side in a rudder shaft slot of a rudder shaft part is put close to one gauge block; the contact of the dial gauge contacts the conical surface of the rubber shaft, the rubber shaft is moved along the surface of the gauge block, and the maximum reading of the dial gauge is recorded; the rubber shaft is turned so the other side in the rubber shaft slot is put close to the gauge block; the contact of the dial gauge contacts the conical surface of the rubber shaft, the rubber shaft is moved along the surface of the gauge block, and the maximum reading of the dial gauge is recorded again; and the absolute value of the difference between the two readings of the dial gauge is the symmetry. The device of the invention has a simple structure and a good application effect. The method of the invention is easy to operate, and costs low.