Abstract: Disclosed is a tele-converter structure with an adjustable back focal length. A lens cylinder is provided with guiding columns protruding radially at an outer wall thereof, and a straight drum is provided with guiding holes corresponding to the guiding columns. An inner rotating ring is provided with an annular body sleeved outside a cylinder body, and the annular body is provided with positioning holes corresponding to the guiding columns. A camera body adapter is fixedly connected at a rear end of the straight drum, and a lens adapter is fixedly connected at a front end of the straight drum. A focusing ring is fitted between the camera body adapter and the lens adapter, and the inner rotating ring is driven to move axially when the focusing ring is rotated.