The invention provides a Brewster law demonstrating and refractive index measuring instrument. The Brewster law demonstrating and refractive index measuring instrument consists of a barrel, a frame 1, a scale plate, a frame 2, angle scale marks, a semicircular optical element, a reflecting light outlet, a photoelectric converter, a polarization detector, a shielding wire, a positioning screw 1, a reflecting outlet cover plate, a transparent panel, a base, a smoke injecting hole, a positioning screw 2, an incident light inlet, a slot, a retaining ring, a laser indicator jack, a laser indicator, a direction adjusting screw, a support, an incident light inlet cover plate, a round hole, a polarization detector slot, a parallel polarization mark, a perpendicular polarization mark and a photoelectric conversion element, wherein the laser indicator is inserted in the retaining ring; the round hole is formed in the longitudinal axis direction of the photoelectric converter; the polarization detector slot is formed in the middle of the photoelectric converter; the polarization detector is inserted in the polarization detector slot; one end of the round hole is sealed by the photoelectric conversion element; and the photoelectric conversion element is connected with a micro-current measurer via the shielding wire.