The invention designs a device for reducing residual amplitude modulation. The device comprises an electro-optical modulation device, three crystals are connected in series in the electro-optical modulation device and are respectively a first cubic crystal, a middle crystal and a second cubic crystal, the middle crystal is an electro-optical modulation crystal and is connected with a high frequency signal source module, the specification of the middle crystal is X mm*Y mm*d mm; the first cubic crystal and the second cubic crystal are connected in series to the two ends of the middle crystal in the X-axis direction, the relative dielectric constants of the first cubic crystal and the second cubic crystal are equal to that of the middle crystal, and the specifications of the first cubic crystal and the second cubic crystal are d/2 mm*Y mm*d mm.