The invention discloses a device and a method for measuring horizontal shear of a support, and relates to a shear measuring device. Double spherical balls are arranged between a top plate and a frame bottom plate, one end of the spherical ball is embedded into a spherical groove of the top plate, the other end of the spherical ball is embedded into a spherical groove of the frame bottom plate, and the double spherical balls enable the top plate to translate freely with a small amplitude in the random horizontal direction; a metal bar is horizontally suspended, one end of the metal bar is fixed on the rim of the frame bottom plate, the other end of the metal bar is contacted with the lateral surface of the top plate, and a strain sheet is stuck to the middle part of the steel bar; an accelerator is arranged on the lateral surface of the top plate; and the top plate translates freely in the random horizontal direction to press the metal bar so that the metal bar is deformed, an axial force of the metal bar is calculated by measuring the strain of the metal bar, and the horizontal shear of the support is obtained. The method can overcome the defects that the conventional measuring method has fussy calibration and only can measure the unidirectional horizontal shear; a horizontal shear vector of the support can be measured by arranging the device between a model and a foundation; and the device and the method are suitable for any structure, do not need calibration, and have low cost.