As the output power of the laser diode (LD) used in the level gauge is small and the reflected and incident beams is are required to be parallel, a cube corner reflector (CCR) is used as reflecting object. There is an inevitable cross shift 2Δℓ of the returning beams, as there is a linear moving error Δℓ of the CCR in vertical direction. The cross shift is the most difficult point for the measuring system. It would cause instability of the instrument, even make it stop working. To get as much light energy as possible, especially from the off-axis beam, into the receiving fiber, some characteristics and parameters of optical fiber are considered, as for example the numerical aperture NA. We designed a special optical antenna used to couple to the receiving fiber. This is a reflecting coaxial multilevel tapered optical antenna used in receiving telescopes. The working principle, design method and performance are introduced. Experimental results indicate, after the optical antenna is set, that even if the light spot drifted off the end of the receiving fiber by several centimeters in cross direction, the signal beam could still enter the receiving fiber. The optical antenna greatly increases the ability to gather the light beam and guarantees the stability of the instrument. Such optical antenna also could be used for laser range finders, Free-Space Optics and other optical fiber system.