A device for assembling a lens into a lens barrel is provided. The device includes a pressing rod and a sensing module positioned above the pressing rod. The pressing rod includes a first end and a second end opposite to the first end. The first end defines a first surface for pressing against the lens. The second end defines a second planar end surface. The sensing module includes a pressing surface for facing toward the second planar end surface, a number of force sensors mounted on the pressing surface, each force sensor being configured for sensing a counterforce applied thereto when the force sensors are pressed against the second planar end surface of the pressing rod.