Abstract: The primary mirror turning device was designed to achieve the 180 revolution according to the special operating requirements of the 4 m SiC primary mirror. Firstly, it was determined that the turning device consisted of main body frame, axial supports and radial supports et al. The axial Whiffle-tree support force and the radial strap force was theoretically analyzed when the primary mirror was in different zenith angles and the strap radial pressure was analyzed when the primary mirror was supported by the strap. Secondary, when the practical structure was designed, the finite element model was built and the rigidity of the main body frame was analyzed, and the mirror's maximum stresses and the turning device's maximum displacements were emphatically studied when the mirror axis was vertical and horizontal. The detailed structure of the whole turning device was designed. Finally, the turning device was machined and assembled according to the designed and analyzed results, and the mirror's impact status was measured when the mirror was turning based on the accelerometer's test. The practical application and the test result indicate that the design requirement of the turning device is achieved, and the direction for similar structure is provided here.