The off-axis reflective optical system with large field-of-view can obtain more abundant information resources, which is the development trend of the future space optical systems. The application of freeform surfaces in off-axis reflective optical systems can increase the field-of-view, but freeform off-axis reflective optical systems with large field-of-view are difficult to design due to fewer initial configurations and complicated optimization processes. In this paper, a design method for the freeform off-axis reflective optical system with large field-of-view is proposed. First, the unobscured freeform initial system with good imaging quality is directly obtained based on the vector aberration theory and the Fermat principle. Then, the final freeform optical system is obtained after simple optimization. This method can reduce the design difficulty of a freeform off-axis reflective optical system with large field-of-view. A freeform off-axis three-mirror optical system with a field-of-view of 30°×3° and an F number of 2 is designed, which verifies the effectiveness of the proposed method.