The invention discloses a device for measuring defect density of an amorphous silicon thin film band gap, which comprises an excitation light source, a dielectric material, a laser probe, a position sensor and a calculating device, wherein the excitation light source emits irradiation light; a to-be-measured sample is immersed in the dielectric material, the irradiation light irradiates on the dielectric material, and thus the refractive index of the dielectric material around the to-be-measured sample is caused to change, and the inner refractive index of the surface thin layer of the to-be-measured sample is caused to change; light signals sent by the laser probe pass through the dielectric material and deflection happens due to the fact that the refractive index of the dielectric material changes; the position sensor senses the deflection and sends deflection signals; and according to the deflection signals, the calculating device calculates the defect density of the measured thin film material. According to the device for measuring the defect density of the amorphous silicon thin film band gap, the defect of the material is tested through measuring a light absorption spectrum of the thin film material, absorption on a low energy end (<1.1eV) has high sensitivity, and defect distribution of the material can be accurately measured.