A lens adjusting device including a frame, a lens, a base, at least a positioning structure, and a locking device is provided. The lens is fixed on the frame. The base has a through hole with a predetermined shape formed thereon. The positioning structure is disposed on the base for movably positioning the frame on a plane above the base. The locking device including a screw, a nut, and a fixer is capable of fixing the frame to the base. The screw penetrates the frame and extends to below the base through the through hole of the base. The size of the through hole is greater than the size of a corresponding cross-section of the screw. The nut is disposed below the base and assembled to the screw to position the screw on the base. The fixer is disposed on the frame to constrain the rotation of the nut.