The space gravitational wave detection is realized by adopting the technology of heterodyne laser interferometry. The accuracy and noise level of the measurement are extremely rigorous. As an important part of space-based gravitational wave observatory, telescope plays the role of laser signal transceiver, and is characterized by high magnification, high image quality, high similarity of wave-front error over the field of view and extraordinary ability to suppress stray light. Aiming at above requirements, methods of design and analysis of the off-axis four-mirror afocal optical system with high magnification are investigated. Based on the theory of primary aberration, the design method of initial structure is explored. The system has an intermediate image plane and an available exit pupil, which facilitates stray light suppression and integration with the scientific interferometer. The wave-front similarity merit function is established. After optimization, the entrance pupil diameter is 200 mm, the magnification is 40. The Root-Mean-Square (RMS) wave-front error is better than 0.005λ and the Peak-to-Valley (PV) value is less than 0.023λ, moreover, the RMS of wave-front similarity residuals are better than 0.000 8λ(λ=1 064 nm) within the ±8 μrad scientific field of view. Over the field of regard for acquisition, the imaging quality is close to the diffraction limit. The tolerance of the system is analyzed and meets the requirements of space-based gravitational wave detection.