A distance measuring device coupled to a camera and a rotator for driving the camera to rotate. The camera includes a photo sensor and a lens. The distance measuring device includes a distance obtaining module, an angle obtaining module, and a computing module coupled to the distance obtaining module and the angle obtaining module. The distance obtaining module is configured to obtain an unaligned target image of a target captured by the camera. A projection of the unaligned target image on a reference plane does not overlap with a projection of a center point of the photo sensor on the reference plane. The reference plane is perpendicular to a rotation axis of the camera. The distance obtaining module is further configured to calculate a projection distance between the projection of the unaligned target image and the projection of the center point. The angle obtaining module is configured to obtain a rotation angle between an unaligned position at which the camera captures the unaligned target image and an aligned position at which the camera captures an aligned target image of the target. A projection of the aligned target image on the reference plane overlapping with the projection of the center point of the photo sensor. The computing module is configured to calculate a target distance between the target and the imaging camera based on the projection distance and the rotation angle.