Two kinds of field rotation occur in large optical telescopes, one is caused by the coordinate transformation between horizontal coordinate system and equatorial coordinate system, the other is caused by the mutual rotation among fold mirrors in telescope. In order to get high spatial resolution and stationary image, field rotation compensation is required. Now K mirror, as a derotator, is widely applied. Using matrix can explain the principal of K mirror and achieve the requirements for derotation, which leads to two sorts of K mirror: the symmetric and asymmetric. Similar to prisms, K mirror can affect the optical axis and imaging.Considered the requirements of machining, assembly and the total optical path, the aperture, volume and base angles of K mirror have been analyzed and optimized in this paper. An example of the symmetric K mirror is designed by Zemax. The tolerance of assembly and alignment have been explained by 3D-geometric relationship and calculated by Zemax and MATLAB.