Aiming at solving the problem of fewer initial structures and complicated optimization processes for freeform surface off-axis reflective optical systems, a method for designing off-axis reflective optical systems based on quantum genetic algorithm was proposed. In such method, quantum genetic algorithm combined with aberration theory was adopted to obtain an initial optical system structure with good image quality, and then the off-axis process was accomplished and the freeform surface was further introduced to obtain the well-designed freeform surface off-axis reflective optical system. The method could easily and quickly provide initial structure with good image quality. Taking the off-axis three-mirror system working in visible light band as an example, a freeform surface off-axis three-mirror system with field-of-view of 10°×4° and F-number of 2 was designed with the primary mirror and the tertiary mirror being integrated together. Simulation results indicate that the designed off-axis three-mirror system exhibits a large field-of-view, a compact structure and a good image quality. The maximum root mean square (RMS) wavefront error of the system is 0.0485 λ(λ=0.6328 μm) and the modulation transfer functions (MTF) of the full fields are all greater than 0.9 at 50 lp/mm. Besides, the integrated structure is benefit for the simplification of the alignment and installation of the system.