An assembly tool for assembling a lens tube in a lens base includes a support base, a fastening unit, a first height sensor, and a second height sensor. The support base defines a positioning groove for receiving the lens base. The fastening unit includes a fastening pole and a torque meter connected to an end of the fastening pole. The fastening pole forms a plurality of connecting portions for engaging with the lens tube. The first height sensor is positioned on the support base to detect a height of the lens base. The second height sensor is positioned on the support base to detect a height of an optical lens formed by the lens tube and the lens base.