Generally， the observation ranges and detection efficiencies of optical remote sensors can be enhanced using wide-field optical imaging systems. Accordingly， the design and manufacturing of freeform-surface-based optical systems have achieved significant progress recently， making the construction of wide-field， rapid， higher-quality-resolution and -imaging， and obstacle-free off-axis reflection systems feasible. The first part of this paper presents a detailed analysis of the aberration characteristics of an off-axis reflection system. This asymmetric higher level aberration increases dramatically as the field of view （FOV）， specifically the meridional FOV， of the system increases. These characteristics place a prominent requirement on image field continuity. The second part of this paper presents a new design approach for two-dimensional （2-D） large FOV optical systems with long focal lengths. First， we built multiple structural forms based on conventional optical systems， leading to a discretized meridional field angle， and with certain specific surfaces of the optical system decomposed into two sub-surfaces. Following this， system optimization was achieved by constraining the system size and improving its structural form. Finally， we designed the off-axis reflective freeform surface optical system. The imaging quality was good within the entire FOV， realizing 2-D large-field imaging of 40°×16°， a focal length of 1 m， and an F-number of 10. The test results prove that the proposed design is good， yielding an optical modulation transfer function of better than 0.26， particularly in the visible spectrum of 400-750 nm with a characteristic frequency of 50 lp/mm.