Structural design and stability analysis of 'phi' 570 mm mirror mount are performed. Under the constraint conditions, the geometric parameters of cross-flexure pivot in large aperture mirror mount are determined. With finite element analysis software (ANSYS), the influences of cross-flexure pivot's setting angle on center drifting and stress distribution of flexure, and the dynamic performance of the whole mirror mount, are analyzed. When the setting angle between the direction of gravity and the setting direction of flexure is 0 or 90 deg., the center drifting of mirror mount is minimum and the stress distribution of flexure is relatively uniform. The nature frequency is 23 Hz, and the maximum amplitude of angular vibration response to random excitation input is 0.91 microrad, which is consonant with experiment results. The performance of mirror mount can satisfy the requirement of precision positioning in Shenguang (SG) II laser system.