The invention discloses a method and device for measuring shale rock mechanical properties and belongs to the field of petroleum and gas. The method comprises carrying out preset mode loading on all three shale core samples of each type of shale core samples belonging to three types to obtain compressive strength, transverse strain curves, axial strain curves and volumetric strain curves of the three shale core samples, wherein the three types of the shale core samples are obtained through shale sampling along three directions comprising a longitudinal direction, a transverse direction and an inclined direction forming an included angle of 60 degrees with the horizontal direction, calculating elasticity moduli of all the shale core samples of three types of the shale core samples according to the volumetric strain curves of the shale core samples, and calculating a Poisson's ratio according to the transverse strain curve, the axial strain curve and the volumetric strain curve. According to shale specificity, through use of multi-directional sampling and a plurality of loading modes, accurate data is acquired. The method and device are suitable for shale cores.