Sparse aperture structure can solve the problems that single-aperture structure with large aperture is difficult to process. It can solve the problem that large aperture is easy to be deformed and can reduce the weight and size of the optical system. The most ideal state of the sparse aperture is to obtain more light information with the smallest light-passing area. The current goal is to obtain the best image quality by researching the arrangement of sparse aperture. However, most of the current sparse aperture structures have the same sub-aperture’s diameter, which leads to a rapid decline of system's modulation transfer function in the mid-frequency band. In this paper, the structure of three sub-apertures surrounding the large sub-aperture called quasi four-mirror structure is proposed through theoretical analysis and MATLAB simulation. The proportion of the diameter of the central mirror of the structure and the diameter of the surrounding sub-aperture is established. This proportional relation can ensure that the actual cutoff frequency is maximized while the filling factor of the entire system is minimized, thereby obtaining higher frequency information. The structure also has a feature that allows uniform acquisition of the mid-frequency information to obtain more detail information of image. The results of imaging simulation show that the imaging quality of the structure is better than that of the four-mirror structure when the filling factor and the light-passing area are equal. The sparse aperture structure of the quasi four-mirror structure proposed in this paper can be applied not only to large-scale astronomical telescopes, but also to medical endoscopes.