Based on the Fraunhofer diffraction theory and a microlens, a high-performance complex optical system for a digital sun sensor is designed . The structure of this optical system is analyzed and the mathematic model is established. The focus, aperture and microlens parameters of the optical system are given and the numeric simulation is carried out. The simulative results show that the energy distribution of the spot formed by the novel complex optical system is concentrated compared with a masking optical system, so the high calculational accuracy of facular centroid is guaranteed. Meanwhile, the sensor with the novel optical system possesses a large field of view and high accuracy.