Stars in the image field are rotating around image center when altaz telescope tracks celestial targets, which increases difficulty to real time target identification and image processing based on multiframe images. The K mirror with large aperture is designed to compensate image rotation for the Coude optical path of altaz telescope in the paper. K mirror is composed of three reflecting mirrors, and its aperture is 42 mm. The angle between reflecting mirror 1 and reflecting mirror 3 is 120 degrees, and K mirror can be used in all spectral ranges. If incoming vector rotates a certain angle, and K mirror rotates half the angle that incoming vector rotated, then output vector does not rotate any angle. The reflecting mirror 1 and reflecting mirror 3 are made of plane mirrors and fixed on a metal triangle, substituting reflector which is made of triangular prism. Every reflecting mirror is assembled by using photoelectric autocollimator and fine rotating platform, so it can be sure that the light axis of K mirror and the rotating axis of K mirror are in one line. The K mirror is directly drived by DC toque motor for the reason of fast response speed, and Renishaw angle encoder which angle resolution can be reach 0.072 arcsec by interpolation is adopted.