An adjustable dual-lens device for 3D stereoscopic surgical microscopes has an outer casing, an image set, and an adjusting set. The image set is mounted in the outer casing and has two lenses. The lenses are pivotally mounted in the outer casing at a spaced interval. The adjusting set is mounted in the outer casing and has two adjusting units. Each adjusting unit has a driving motor, a cam, and a limiting element. The driving motor is mounted in the outer casing adjacent to one of the lenses and has a driving shaft. The cam is eccentrically mounted around the driving shaft and is pressed against the lens that is adjacent to the driving motor. The limiting element is connected to the outer casing and the corresponding lens to enable the corresponding lens to press against the cam.