The invention relates to a device and method for testing the torsional rigidity and mode of shafting, and belongs to the field of rigidity and mode test. In the device, a mass disc is bonded to the free end of a shaft to be tested, additional square blocks are bonded to the two sides, opposite to each other, in the side arm of the shaft to be tested respectively, and accelerometers are mounted onthe additional square blocks respectively. The device is easy to realize, convenient to operate and high in test precision, and fills the blanks in the field of torsional rigidity and mode test of theshafting. According to the method, the characteristic that the additional square blocks can act torsional moment excitation and test response of the tested shaft in the torsional direction is used, the mass disc is used to change the mass characteristic of the shaft, the torsional rigidity and moment of inertia of the tested shaft are calculated, the torsional rigidity and mode measured by the method can reflect the torsional oscillation characteristic of the shafting visually, and the method further has the advantage of high precision.