Stereo system plays an important role in the navigation of MAVs. In this paper, we design a single-camera catadioptric omnistereo system for MAV, which consists of one hyperboloidal mirror, one hyperboloidal-planar combined mirror, and one conventional camera. System parameters are optimized based on the analysis of constraints and each parameter's influence on performance. Projective model of this system is derived, which provides a foundation for sphere-based calibration algorithm. It calibrates not only the conventional camera parameters, but also the mirror parameters. We also prove that a minimum of two spheres are needed to calibrate the seven parameters.