To meet the demand of timely panoramic reconnaissance by combat robot, by analyzing characteristics of the present unwrapping methods of panoramic image and imaging mapping relation of catadioptric vision reconnaissance system for combat robot, a fast low distortion unwrapping algorithm of panoramic image was presented. On the base of approximate unwrapping algorithm of concentric rings, the algorithm unwrapped panoramic annular image radically by the beam path tracking mapping to ensure the unwrapping precision, and the table look-up method and coordinate rotation digital computer (CORDIC) was adopted to improve calculation speed. The relation of lengthwise and transverse constraint was presented to avoid the portrait and transverse proportion distortion of unwrapped image. Experiments indicate the proposed algorithm is valid.